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/syntax.c

@@ -40,10 +40,12 @@
 #include "nvim/option.h"
 #include "nvim/os_unix.h"
 #include "nvim/path.h"
+#include "nvim/macros.h"
 #include "nvim/regexp.h"
 #include "nvim/screen.h"
 #include "nvim/strings.h"
 #include "nvim/syntax_defs.h"
+#include "nvim/terminal.h"
 #include "nvim/ui.h"
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"
@@ -6636,7 +6638,7 @@ static garray_T attr_table = GA_EMPTY_INIT_VALUE;
  * if the combination is new.
  * Return 0 for error.
  */
-static int get_attr_entry(attrentry_T *aep)
+int get_attr_entry(attrentry_T *aep)
 {
   garray_T *table = &attr_table;
   attrentry_T *taep;
@@ -7424,7 +7426,6 @@ char_u *get_highlight_name(expand_T *xp, int idx)
   return HL_TABLE()[idx].sg_name;
 }
 
-#define RGB(r, g, b) ((r << 16) | (g << 8) | b)
 color_name_table_T color_name_table[] = {
   // Color names taken from
   // http://www.rapidtables.com/web/color/RGB_Color.htm