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/extension_cli/src/main.rs

@@ -9,7 +9,7 @@ use std::{
 use ::fs::{copy_recursive, CopyOptions, Fs, RealFs};
 use anyhow::{anyhow, bail, Context, Result};
 use clap::Parser;
-use extension::{
+use extension_host::{
     extension_builder::{CompileExtensionOptions, ExtensionBuilder},
     ExtensionManifest,
 };