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/fileio.c

@@ -4756,10 +4756,11 @@ buf_check_timestamp (
   char_u      *s;
   char        *reason;
 
-  /* If there is no file name, the buffer is not loaded, 'buftype' is
-   * set, we are in the middle of a save or being called recursively: ignore
-   * this buffer. */
-  if (buf->b_ffname == NULL
+  // If its a terminal, there is no file name, the buffer is not loaded,
+  // 'buftype' is set, we are in the middle of a save or being called
+  // recursively: ignore this buffer.
+  if (buf->terminal
+      || buf->b_ffname == NULL
       || buf->b_ml.ml_mfp == NULL
       || *buf->b_p_bt != NUL
       || buf->b_saving
@@ -5208,6 +5209,7 @@ static struct event_name {
   {"TabNew",          EVENT_TABNEW},
   {"TabNewEntered",   EVENT_TABNEWENTERED},
   {"TermChanged",     EVENT_TERMCHANGED},
+  {"TermOpen",        EVENT_TERMOPEN},
   {"TermResponse",    EVENT_TERMRESPONSE},
   {"TextChanged",     EVENT_TEXTCHANGED},
   {"TextChangedI",    EVENT_TEXTCHANGEDI},