debugger: Rework language association with the debuggers (#29945)

- Languages now define their preferred debuggers in `config.toml`.
- `LanguageRegistry` now exposes language config even for languages that
are not yet loaded. This necessitated extension registry changes (we now
deserialize config.toml of all language entries when loading new
extension index), but it should be backwards compatible with the old
format. /cc @maxdeviant

Release Notes:

- N/A

---------

Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Remco Smits <djsmits12@gmail.com>
Co-authored-by: Anthony <anthony@zed.dev>

crates/languages/src/python/config.toml

@@ -29,3 +29,4 @@ brackets = [
 auto_indent_using_last_non_empty_line = false
 increase_indent_pattern = "^[^#].*:\\s*$"
 decrease_indent_pattern = "^\\s*(else|elif|except|finally)\\b.*:"
+debuggers = ["Debugpy"]