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/memline.c

@@ -278,10 +278,11 @@ int ml_open(buf_T *buf)
   /*
    * When 'updatecount' is non-zero swap file may be opened later.
    */
-  if (p_uc && buf->b_p_swf)
+  if (!buf->terminal && p_uc && buf->b_p_swf) {
     buf->b_may_swap = true;
-  else
+  } else {
     buf->b_may_swap = false;
+  }
 
   /*
    * Open the memfile.  No swap file is created yet.
@@ -488,7 +489,8 @@ void ml_open_file(buf_T *buf)
   char_u      *dirp;
 
   mfp = buf->b_ml.ml_mfp;
-  if (mfp == NULL || mfp->mf_fd >= 0 || !buf->b_p_swf || cmdmod.noswapfile) {
+  if (mfp == NULL || mfp->mf_fd >= 0 || !buf->b_p_swf || cmdmod.noswapfile
+      || buf->terminal) {
     return; /* nothing to do */
   }