Add new extension crate (#20089)

This PR adds a new `extension` crate, containing some contents extracted
from the `extension_host`.

Right now it contains just the `ExtensionManifest` and
`ExtensionBuilder`, although we may move more of the extension interface
into here.

The introduction of the `extension` crate allows us to depend on it in
the `extension_cli`, thereby eliminating the need for the `no-webrtc`
feature on a number of crates.

Release Notes:

- N/A

crates/extension/src/extension.rs

@@ -0,0 +1,50 @@
+pub mod extension_builder;
+mod extension_manifest;
+
+use anyhow::{anyhow, bail, Context as _, Result};
+use semantic_version::SemanticVersion;
+
+pub use crate::extension_manifest::*;
+
+pub fn parse_wasm_extension_version(
+    extension_id: &str,
+    wasm_bytes: &[u8],
+) -> Result<SemanticVersion> {
+    let mut version = None;
+
+    for part in wasmparser::Parser::new(0).parse_all(wasm_bytes) {
+        if let wasmparser::Payload::CustomSection(s) =
+            part.context("error parsing wasm extension")?
+        {
+            if s.name() == "zed:api-version" {
+                version = parse_wasm_extension_version_custom_section(s.data());
+                if version.is_none() {
+                    bail!(
+                        "extension {} has invalid zed:api-version section: {:?}",
+                        extension_id,
+                        s.data()
+                    );
+                }
+            }
+        }
+    }
+
+    // The reason we wait until we're done parsing all of the Wasm bytes to return the version
+    // is to work around a panic that can happen inside of Wasmtime when the bytes are invalid.
+    //
+    // By parsing the entirety of the Wasm bytes before we return, we're able to detect this problem
+    // earlier as an `Err` rather than as a panic.
+    version.ok_or_else(|| anyhow!("extension {} has no zed:api-version section", extension_id))
+}
+
+fn parse_wasm_extension_version_custom_section(data: &[u8]) -> Option<SemanticVersion> {
+    if data.len() == 6 {
+        Some(SemanticVersion::new(
+            u16::from_be_bytes([data[0], data[1]]) as _,
+            u16::from_be_bytes([data[2], data[3]]) as _,
+            u16::from_be_bytes([data[4], data[5]]) as _,
+        ))
+    } else {
+        None
+    }
+}