[Flight Parcel] Implement findSourceMapURL (#32294)

This implements `findSourceMapURL` in react-server-dom-parcel, enabling
source maps for replayed server errors on the client. It utilizes a new
endpoint in the Parcel dev server that returns the source map for a
given bundle/file. The error overlay UI has also been updated to handle
these stacks. See https://github.com/parcel-bundler/parcel/pull/10082

Also updated the fixture to the latest Parcel canary. A few APIs have
changed. We do have a higher level library wrapper now (`@parcel/rsc`
added in https://github.com/parcel-bundler/parcel/pull/10074) but I left
the fixture using the lower level APIs directly here since it is easier
to see how react-server-dom-parcel is used.

scripts/flow/environment.js

@@ -109,6 +109,7 @@ declare var parcelRequire: {
   extendImportMap: (importMap: {[string]: string}) => void,
   meta: {
     publicUrl: string,
+    devServer: string | null,
   },
 };