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, 10 deletions

diff --git a/src/nvim/quickfix.c b/src/nvim/quickfix.c
index 39117b262a..8e55cced78 100644
--- a/src/nvim/quickfix.c
+++ b/src/nvim/quickfix.c
@@ -2377,7 +2377,6 @@ static void qf_fill_buffer(qf_info_T *qi)
   KeyTyped = old_KeyTyped;
 }
 
-
 /*
  * Return TRUE if "buf" is the quickfix buffer.
  */
@@ -2386,22 +2385,18 @@ int bt_quickfix(buf_T *buf)
   return buf != NULL && buf->b_p_bt[0] == 'q';
 }
 
-/*
- * Return TRUE if "buf" is a "nofile" or "acwrite" buffer.
- * This means the buffer name is not a file name.
- */
+// Return TRUE if "buf" is a "nofile", "acwrite" or "terminal" buffer.
+// This means the buffer name is not a file name.
 int bt_nofile(buf_T *buf)
 {
   return buf != NULL && ((buf->b_p_bt[0] == 'n' && buf->b_p_bt[2] == 'f')
-                         || buf->b_p_bt[0] == 'a');
+                         || buf->b_p_bt[0] == 'a' || buf->terminal);
 }
 
-/*
- * Return TRUE if "buf" is a "nowrite" or "nofile" buffer.
- */
+// Return TRUE if "buf" is a "nowrite", "nofile" or "terminal" buffer.
 int bt_dontwrite(buf_T *buf)
 {
-  return buf != NULL && buf->b_p_bt[0] == 'n';
+  return buf != NULL && (buf->b_p_bt[0] == 'n' || buf->terminal);
 }
 
 int bt_dontwrite_msg(buf_T *buf)