Extract Astro support into an extension (#9835)

This PR extracts Astro support into an extension and removes the
built-in Astro support from Zed.

Release Notes:

- Removed built-in support for Astro, in favor of making it available as
an extension. The Astro extension will be suggested for download when
you open a `.astro` file.

crates/extensions_ui/src/extension_suggest.rs

@@ -17,6 +17,7 @@ pub fn suggested_extension(file_extension_or_name: &str) -> Option<Arc<str>> {
     SUGGESTED
         .get_or_init(|| {
             [
+                ("astro", "astro"),
                 ("beancount", "beancount"),
                 ("dockerfile", "Dockerfile"),
                 ("elisp", "el"),