Extension API: Support for external formatters other binaries

[notpeter]

Summary

Support extensions downloading and defaulting to an external formatter binary.

Description

Extensions can download a language server, which may provide a formatter for a language. They may also specify prettier_plugins / prettier_parser_name in a config.toml for a language.

But extensions cannot download a github release binary / npm package and provide settings to define use as a default formatter for a given language.

Links:

• https://github.com/zed-extensions/lua/issues/4 (Stylua as default Lua formatter)
• https://github.com/zed-industries/zed/pull/28727#issuecomment-2831003042
• https://github.com/zed-extensions/bash/issues/14