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/fold.c

@@ -137,7 +137,7 @@ void copyFoldingState(win_T *wp_from, win_T *wp_to)
 int hasAnyFolding(win_T *win)
 {
   /* very simple now, but can become more complex later */
-  return win->w_p_fen
+  return !win->w_buffer->terminal && win->w_p_fen
          && (!foldmethodIsManual(win) || !GA_EMPTY(&win->w_folds));
 }
 
@@ -768,6 +768,9 @@ void clearFolding(win_T *win)
 void foldUpdate(win_T *wp, linenr_T top, linenr_T bot)
 {
   fold_T      *fp;
+  if (wp->w_buffer->terminal) {
+    return;
+  }
 
   /* Mark all folds from top to bot as maybe-small. */
   (void)foldFind(&curwin->w_folds, top, &fp);