Add language-agnostic snippets (#13253)

Note that right now we can't attach a language server to arbitrary
buffer, which is why I've listed a bunch of languages verbatim.
See
https://github.com/zed-industries/simple-completion-language-server/tree/main
for docs on how to define your snippets. They should be placed in
~/.config/zed/snippets ; `snippets.(toml|json)` file can be used to
define language-agnostic snippets, and any other name (e.g.
`python.toml`) will apply only to buffers of that particular type.

There's https://github.com/rafamadriz/friendly-snippets you can use as a
repository of snippets, for your convenience.

Fixes https://github.com/zed-industries/zed/issues/4611

Release Notes:
- Added support for snippets via simple-completion-language-server