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/Cargo.toml

@@ -1,67 +0,0 @@
-[package]
-name = "extension"
-version = "0.1.0"
-edition = "2021"
-publish = false
-license = "GPL-3.0-or-later"
-
-[lints]
-workspace = true
-
-[lib]
-path = "src/extension_store.rs"
-doctest = false
-
-[features]
-no-webrtc = ["workspace/no-webrtc"]
-
-[dependencies]
-anyhow.workspace = true
-assistant_slash_command.workspace = true
-async-compression.workspace = true
-async-tar.workspace = true
-async-trait.workspace = true
-client.workspace = true
-collections.workspace = true
-fs.workspace = true
-futures.workspace = true
-gpui.workspace = true
-http_client.workspace = true
-indexed_docs.workspace = true
-language.workspace = true
-log.workspace = true
-lsp.workspace = true
-node_runtime.workspace = true
-paths.workspace = true
-project.workspace = true
-release_channel.workspace = true
-schemars.workspace = true
-semantic_version.workspace = true
-serde.workspace = true
-serde_json.workspace = true
-serde_json_lenient.workspace = true
-settings.workspace = true
-snippet_provider.workspace = true
-task.workspace = true
-theme.workspace = true
-toml.workspace = true
-ui.workspace = true
-url.workspace = true
-util.workspace = true
-wasm-encoder.workspace = true
-wasmparser.workspace = true
-wasmtime-wasi.workspace = true
-wasmtime.workspace = true
-wit-component.workspace = true
-workspace.workspace = true
-
-[dev-dependencies]
-ctor.workspace = true
-env_logger.workspace = true
-fs = { workspace = true, features = ["test-support"] }
-gpui = { workspace = true, features = ["test-support"] }
-language = { workspace = true, features = ["test-support"] }
-parking_lot.workspace = true
-project = { workspace = true, features = ["test-support"] }
-reqwest_client.workspace = true
-workspace = { workspace = true, features = ["test-support"] }