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/ex_cmds.lua

@@ -2235,6 +2235,11 @@ return {
     flags=bit.bor(NEEDARG, EXTRA, TRLBAR, NOTRLCOM, CMDWIN),
     func='ex_tearoff',
   },
+  {
+    command='terminal',
+    flags=bit.bor(BANG, FILES, CMDWIN),
+    func='ex_terminal',
+  },
   {
     command='tfirst',
     flags=bit.bor(RANGE, NOTADR, BANG, TRLBAR, ZEROR),
@@ -2255,6 +2260,16 @@ return {
     flags=bit.bor(BANG, TRLBAR),
     func='ex_tag',
   },
+  {
+    command='tmap',
+    flags=bit.bor(EXTRA, TRLBAR, NOTRLCOM, USECTRLV, CMDWIN),
+    func='ex_map',
+  },
+  {
+    command='tmapclear',
+    flags=bit.bor(EXTRA, TRLBAR, CMDWIN),
+    func='ex_mapclear',
+  },
   {
     command='tmenu',
     flags=bit.bor(RANGE, NOTADR, ZEROR, EXTRA, TRLBAR, NOTRLCOM, USECTRLV, CMDWIN),
@@ -2265,6 +2280,11 @@ return {
     flags=bit.bor(RANGE, NOTADR, BANG, TRLBAR, ZEROR),
     func='ex_tag',
   },
+  {
+    command='tnoremap',
+    flags=bit.bor(EXTRA, TRLBAR, NOTRLCOM, USECTRLV, CMDWIN),
+    func='ex_map',
+  },
   {
     command='topleft',
     flags=bit.bor(NEEDARG, EXTRA, NOTRLCOM),
@@ -2290,6 +2310,11 @@ return {
     flags=bit.bor(BANG, TRLBAR, WORD1),
     func='ex_tag',
   },
+  {
+    command='tunmap',
+    flags=bit.bor(EXTRA, TRLBAR, NOTRLCOM, USECTRLV, CMDWIN),
+    func='ex_unmap',
+  },
   {
     command='tunmenu',
     flags=bit.bor(EXTRA, TRLBAR, NOTRLCOM, USECTRLV, CMDWIN),