terminal: New module that implements a terminal emulator

This commit integrates libvterm with Neovim and implements a terminal emulator
with nvim buffers as the display mechanism. Terminal buffers can be created
using any of the following methods:

- Opening a file with name following the "term://[${cwd}//[${pid}:]]${cmd}"
  URI pattern where:
  - cwd is the working directory of the process
  - pid is the process id. This is just for use in session files where a pid
    would have been assigned to the saved buffer title.
  - cmd is the command to run
- Invoking the `:terminal` ex command
- Invoking the `termopen` function which returns a job id for automating the
  terminal window.

Some extra changes were also implemented to adapt with terminal buffers. Here's
an overview:

- The `main` function now sets a BufReadCmd autocmd to intercept the term:// URI
  and spawn the terminal buffer instead of reading the file.
- terminal buffers behave as if the following local buffer options were set:
  - `nomodifiable`
  - `swapfile`
  - `undolevels=-1`
  - `bufhidden=hide`
- All commands that delete buffers(`:bun`, `:bd` and `:bw`) behave the same for
  terminal buffers, but only work when bang is passed(eg: `:bwipeout!`)
- A new "terminal" mode was added. A consequence is that a new set of mapping
  commands were implemented with the "t" prefix(tmap, tunmap, tnoremap...)
- The `edit` function(which enters insert mode) will actually enter terminal
  mode if the current buffer is a terminal
- The `put` operator was adapted to send data to the terminal instead of
  modifying the buffer directly.
- A window being resized will also trigger a terminal resize if the window
  displays the terminal.

src/nvim/buffer.c

@@ -68,6 +68,7 @@
 #include "nvim/spell.h"
 #include "nvim/strings.h"
 #include "nvim/syntax.h"
+#include "nvim/terminal.h"
 #include "nvim/ui.h"
 #include "nvim/undo.h"
 #include "nvim/version.h"
@@ -307,20 +308,28 @@ close_buffer (
   bool del_buf = (action == DOBUF_DEL || action == DOBUF_WIPE);
   bool wipe_buf = (action == DOBUF_WIPE);
 
-  /*
-   * Force unloading or deleting when 'bufhidden' says so.
-   * The caller must take care of NOT deleting/freeing when 'bufhidden' is
-   * "hide" (otherwise we could never free or delete a buffer).
-   */
-  if (buf->b_p_bh[0] == 'd') {          /* 'bufhidden' == "delete" */
-    del_buf = true;
+  // Force unloading or deleting when 'bufhidden' says so, but not for terminal
+  // buffers.
+  // The caller must take care of NOT deleting/freeing when 'bufhidden' is
+  // "hide" (otherwise we could never free or delete a buffer).
+  if (!buf->terminal) {
+    if (buf->b_p_bh[0] == 'd') {         // 'bufhidden' == "delete"
+      del_buf = true;
+      unload_buf = true;
+    } else if (buf->b_p_bh[0] == 'w') {  // 'bufhidden' == "wipe"
+      del_buf = true;
+      unload_buf = true;
+      wipe_buf = true;
+    } else if (buf->b_p_bh[0] == 'u')    // 'bufhidden' == "unload"
+      unload_buf = true;
+  }
+
+  if (buf->terminal && (unload_buf || del_buf || wipe_buf)) {
+    // terminal buffers can only be wiped
     unload_buf = true;
-  } else if (buf->b_p_bh[0] == 'w') { /* 'bufhidden' == "wipe" */
     del_buf = true;
-    unload_buf = true;
     wipe_buf = true;
-  } else if (buf->b_p_bh[0] == 'u')     /* 'bufhidden' == "unload" */
-    unload_buf = true;
+  }
 
   if (win_valid(win)) {
     /* Set b_last_cursor when closing the last window for the buffer.
@@ -383,6 +392,10 @@ close_buffer (
   if (buf->b_nwindows > 0 || !unload_buf)
     return;
 
+  if (buf->terminal) {
+    terminal_close(buf->terminal, NULL);
+  } 
+
   /* Always remove the buffer when there is no file name. */
   if (buf->b_ffname == NULL)
     del_buf = TRUE;
@@ -925,8 +938,8 @@ do_buffer (
     if (action != DOBUF_WIPE && buf->b_ml.ml_mfp == NULL && !buf->b_p_bl)
       return FAIL;
 
-    if (!forceit && bufIsChanged(buf)) {
-      if ((p_confirm || cmdmod.confirm) && p_write) {
+    if (!forceit && (buf->terminal || bufIsChanged(buf))) {
+      if ((p_confirm || cmdmod.confirm) && p_write && !buf->terminal) {
         dialog_changed(buf, FALSE);
         if (!buf_valid(buf))
           /* Autocommand deleted buffer, oops!  It's not changed
@@ -937,9 +950,14 @@ do_buffer (
         if (bufIsChanged(buf))
           return FAIL;
       } else {
-        EMSGN(_("E89: No write since last change for buffer %" PRId64
-                " (add ! to override)"),
-              buf->b_fnum);
+        if (buf->terminal) {
+          EMSG2(_("E89: %s will be killed(add ! to override)"),
+              (char *)buf->b_fname);
+        } else {
+          EMSGN(_("E89: No write since last change for buffer %" PRId64
+                  " (add ! to override)"),
+                buf->b_fnum);
+        }
         return FAIL;
       }
     }