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, 3 insertions, 2 deletions

diff --git a/src/nvim/vim.h b/src/nvim/vim.h
index a4ce9084c2..12b73843f8 100644
--- a/src/nvim/vim.h
+++ b/src/nvim/vim.h
@@ -112,9 +112,10 @@ Error: configure did not run properly.Check auto/config.log.
 #define SHOWMATCH       (0x700 + INSERT) /* show matching paren */
 #define CONFIRM         0x800   /* ":confirm" prompt */
 #define SELECTMODE      0x1000  /* Select mode, only for mappings */
+#define TERM_FOCUS      0x2000  // Terminal focus mode
 
-#define MAP_ALL_MODES   (0x3f | SELECTMODE)     /* all mode bits used for
-                                                 * mapping */
+// all mode bits used for mapping
+#define MAP_ALL_MODES   (0x3f | SELECTMODE | TERM_FOCUS)
 
 /* directions */
 #define FORWARD                 1