support multiple fuzzer instances in build system web interface #24652
Description
Split from #24588.
The build system web interface currently only supports a single fuzz test step. It must be enhanced to show the user all steps with fuzz tests, and allow the user to view information on any of them.
Because the fuzzer UI includes an in-browser source code view, fixing this issue properly will require enhancing that logic -- currently in lib/docs/wasm/Walk.zig -- to avoid global state, instead allowing separate instances of the Zig source code state to be created.