Create an online editor for people to try out Fe

### What is wrong?

This doesn't strictly belong here but for the lack of a better place and to make sure to remember it, I'll file it here as a documentation issue.

We want an online playground for people to try out Fe similar to e.g. https://play.rust-lang.org for Rust.
@pipermerriam recently pointed out that it makes sense to prioritize this and get something up early even at a stage where the language isn't yet fully functional.

### How can it be fixed

There are generally two possible ways:

1. Frontend + Backend where the frontend sends code over to a backend that runs the code and returns the compiler output back to the frontend

2. Compile the compiler to WASM and have it sit in the frontend directly, eliminating the need for a backend.

No files, tests, or entry points are named. Compare the two proposed approaches—frontend plus backend, or compiling the compiler to WASM—using the Rust playground as a reference. Done means users can enter Fe code online and receive compiler output.

Written by the indexing model from the issue text.