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:

• Formatter not working zed-extensions/lua#4 (Stylua as default Lua formatter)
• Change default formatter settings for LaTeX #28727 (comment)
• Add bundled shellcheck binary zed-extensions/bash#14