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/buffer_defs.h

@@ -27,7 +27,7 @@
 // for String
 #include "nvim/api/private/defs.h"
 
-#define MODIFIABLE(buf) (buf->b_p_ma)
+#define MODIFIABLE(buf) (!buf->terminal && buf->b_p_ma)
 
 /*
  * Flags for w_valid.
@@ -103,6 +103,9 @@ typedef struct file_buffer buf_T;       /* forward declaration */
 // for FileID
 #include "nvim/os/fs_defs.h"
 
+// for Terminal
+#include "nvim/terminal.h"
+
 /*
  * The taggy struct is used to store the information about a :tag command.
  */
@@ -751,6 +754,8 @@ struct file_buffer {
                                  * may use a different synblock_T. */
 
   signlist_T *b_signlist;       /* list of signs to draw */
+
+  Terminal *terminal;           // Terminal instance associated with the buffer
 };
 
 /*