Rename extension crate to extension_host (#20081)

This PR renames the `extension` crate to `extension_host`.

This is to free up the name so that we can create a smaller-scoped
`extension` crate.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -39,7 +39,7 @@ db.workspace = true
 diagnostics.workspace = true
 editor.workspace = true
 env_logger.workspace = true
-extension.workspace = true
+extension_host.workspace = true
 extensions_ui.workspace = true
 feature_flags.workspace = true
 feedback.workspace = true