Allow inserting text into the editor via the action (#16131)

Improves workflows, based on the keymaps that group actions behind a
certain symbol.
E.g.

```json5
", o k": "zed::OpenKeymap",
", o K": "zed::OpenDefaultKeymap",
// other `,`-based keymaps
```

Now, it's possible to do 
```json
", ,": ["editor::HandleInput", ","]
```

and type `,` without waiting for the timeout due to `,`-based bindings.

Release Notes:

- Add an `editor::HandleInput` action to ease typing symbols that are
part of keymaps. E.g. if `, o k` keybinding is bound, `", ,":
["editor::HandleInput", ","]` would allow to type `,` without timeouts.

---------

Co-authored-by: Alex Kladov <aleksey.kladov@gmail.com>