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/getchar.c

@@ -2538,6 +2538,7 @@ fix_input_buffer (
  * for :xmap  mode is VISUAL
  * for :smap  mode is SELECTMODE
  * for :omap  mode is OP_PENDING
+ * for :tmap  mode is TERM_FOCUS
  *
  * for :abbr  mode is INSERT + CMDLINE
  * for :iabbr mode is INSERT
@@ -3056,6 +3057,8 @@ int get_map_mode(char_u **cmdp, int forceit)
     mode = SELECTMODE;                          /* :smap */
   else if (modec == 'o')
     mode = OP_PENDING;                          /* :omap */
+  else if (modec == 't')
+    mode = TERM_FOCUS;                            // :tmap
   else {
     --p;
     if (forceit)
@@ -3923,6 +3926,9 @@ makemap (
         case LANGMAP:
           c1 = 'l';
           break;
+        case TERM_FOCUS:
+          c1 = 't';
+          break;
         default:
           EMSG(_("E228: makemap: Illegal mode"));
           return FAIL;