Extract Terraform extension (#10479)

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

Release Notes:

- Removed built-in support for Terraform, in favor of making it
available as
an extension. The Terraform extension will be suggested for download
when you
open a `.tf`, `.tfvars`, or `.hcl` file.

crates/extensions_ui/src/extension_suggest.rs

@@ -59,6 +59,7 @@ const SUGGESTIONS_BY_EXTENSION_ID: &[(&str, &[&str])] = &[
     ("svelte", &["svelte"]),
     ("swift", &["swift"]),
     ("templ", &["templ"]),
+    ("terraform", &["tf", "tfvars", "hcl"]),
     ("toml", &["Cargo.lock", "toml"]),
     ("wgsl", &["wgsl"]),
     ("zig", &["zig"]),