CI: Add manual workflow dispatch to JS/WASM artifacts build

This allows us to manually kick off jobs for the JS/WASM artifacts
build with an explicit reference to build, which is useful if we want to
generate benchmark results for older commits.

Note that this workflow does not actually produce benchmark results -
that's the job of another workflow. The webhook storing the benchmark
results only appends new benchmarks; existing ones will be kept.

Also note that the GitHub checkout action defaults to the default
reference if `inputs.reference_to_build` is not provided, such as in the
case of a push to the master branch.

.github/workflows/js-and-wasm-artifacts.yml

@@ -3,6 +3,12 @@ name: Package the js repl as a binary artifact
 on:
   push:
     branches: [master]
+  workflow_dispatch:
+    inputs:
+      reference_to_build:
+        description: 'Branch, tag or commit hash to build'
+        required: true
+        type: string
 
 env:
   LADYBIRD_SOURCE_DIR: ${{ github.workspace }}
@@ -40,6 +46,8 @@ jobs:
     steps:
       - name: Checkout LadybirdBrowser/ladybird
         uses: actions/checkout@v5
+        with:
+          ref: ${{ inputs.reference_to_build }}
 
       - name: "Set up environment"
         uses: ./.github/actions/setup