Extract HTML support into an extension (#10130)

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

Release Notes:

- Removed built-in support for HTML, in favor of making it available as
an extension. The HTML extension will be suggested for download when you
open a `.html`, `.htm`, or `.shtml` file.

extensions/html/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "zed_html"
+version = "0.0.1"
+edition = "2021"
+publish = false
+license = "Apache-2.0"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/html.rs"
+crate-type = ["cdylib"]
+
+[dependencies]
+zed_extension_api = "0.0.4"