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

extensions/snippets/extension.toml

@@ -0,0 +1,12 @@
+id = "snippets"
+name = "Snippets"
+description = "Support for language-agnostic snippets, provided by simple-completion-language-server"
+version = "0.0.1"
+schema_version = 1
+authors = []
+repository = "https://github.com/zed-extensions/svelte"
+
+[language_servers.snippet-completion-server]
+name = "Snippet Completion Server"
+languages = ["TypeScript", "TSX", "JavaScript", "JSDoc", "Go", "Markdown", "Rust", "C", "C++", "PHP", "Python", "Ruby", "Shell"]
+language_ids = { "TypeScript" = "typescript", "TSX" = "typescriptreact", "JavaScript" = "javascript" }