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, 5 deletions

diff --git a/src/nvim/os/event.c b/src/nvim/os/event.c
index 92a5205eaf..fd9ff5b230 100644
--- a/src/nvim/os/event.c
+++ b/src/nvim/os/event.c
@@ -20,6 +20,7 @@
 #include "nvim/misc2.h"
 #include "nvim/ui.h"
 #include "nvim/screen.h"
+#include "nvim/terminal.h"
 
 #include "nvim/lib/klist.h"
 
@@ -63,6 +64,7 @@ void event_init(void)
   // finish mspgack-rpc initialization
   channel_init();
   server_init();
+  terminal_init();
 }
 
 void event_teardown(void)
@@ -83,6 +85,7 @@ void event_teardown(void)
   job_teardown();
   server_teardown();
   signal_teardown();
+  terminal_teardown();
   // this last `uv_run` will return after all handles are stopped, it will
   // also take care of finishing any uv_close calls made by other *_teardown
   // functions.
@@ -169,11 +172,6 @@ void event_push(Event event, bool deferred)
 void event_process(void)
 {
   process_events_from(deferred_events);
-
-  if (must_redraw) {
-    update_screen(0);
-    ui_flush();
-  }
 }
 
 static void process_events_from(klist_t(Event) *queue)