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.

1 files changed, 5 insertions, 3 deletions

diff --git a/src/nvim/option.c b/src/nvim/option.c
index 19202030c6..3f12709521 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -1746,7 +1746,7 @@ static char *(p_scbopt_values[]) = {"ver", "hor", "jump", NULL};
 static char *(p_debug_values[]) = {"msg", "throw", "beep", NULL};
 static char *(p_ead_values[]) = {"both", "ver", "hor", NULL};
 static char *(p_buftype_values[]) =
-{"nofile", "nowrite", "quickfix", "help", "acwrite", NULL};
+{"nofile", "nowrite", "quickfix", "help", "acwrite", "terminal", NULL};
 static char *(p_bufhidden_values[]) = {"hide", "unload", "delete", "wipe", NULL};
 static char *(p_bs_values[]) = {"indent", "eol", "start", NULL};
 static char *(p_fdm_values[]) = {"manual", "expr", "marker", "indent", "syntax",
@@ -4097,9 +4097,11 @@ did_set_string_option (
   }
   /* When 'buftype' is set, check for valid value. */
   else if (gvarp == &p_bt) {
-    if (check_opt_strings(curbuf->b_p_bt, p_buftype_values, FALSE) != OK)
+    if ((curbuf->terminal && curbuf->b_p_bt[0] != 't')
+        || (!curbuf->terminal && curbuf->b_p_bt[0] == 't')
+        || check_opt_strings(curbuf->b_p_bt, p_buftype_values, FALSE) != OK) {
       errmsg = e_invarg;
-    else {
+    } else {
       if (curwin->w_status_height) {
         curwin->w_redr_status = TRUE;
         redraw_later(VALID);