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/terminal.h

@@ -0,0 +1,33 @@
+#ifndef NVIM_TERMINAL_H
+#define NVIM_TERMINAL_H
+
+#include <stddef.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+typedef struct terminal Terminal;
+typedef void (*terminal_write_cb)(char *buffer, size_t size, void *data);
+typedef void (*terminal_resize_cb)(uint16_t width, uint16_t height, void *data);
+typedef void (*terminal_close_cb)(void *data);
+
+typedef struct {
+  void *data;
+  uint16_t width, height;
+  terminal_write_cb write_cb;
+  terminal_resize_cb resize_cb;
+  terminal_close_cb close_cb;
+} TerminalOptions;
+
+#define TERMINAL_OPTIONS_INIT ((TerminalOptions) {           \
+    .data = NULL,                                            \
+    .width = 80,                                             \
+    .height = 24,                                            \
+    .write_cb = NULL,                                        \
+    .resize_cb = NULL,                                       \
+    .close_cb = NULL                                         \
+    })
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "terminal.h.generated.h"
+#endif
+#endif  // NVIM_TERMINAL_H