Make GOOS=js GOARCH=wasm a possible build target

[bahner]

As writing files is not really applicable in WebAssembly create a noop filewriter to make compilation possible.

[welcome]

Thank you for submitting this PR!
A maintainer will be here shortly to review it.
We are super grateful, but we are also overloaded! Help us by making sure that:

• The context for this PR is clear, with relevant discussion, decisions
  and stakeholders linked/mentioned.

• Your contribution itself is clear (code comments, self-review for the
  rest) and in its best form. Follow the code contribution
  guidelines
  if they apply.

Getting other community members to do a review would be great help too on complex PRs (you can ask in the chats/forums). If you are unsure about something, just leave us a comment.
Next steps:

• A maintainer will triage and assign priority to this PR, commenting on
  any missing things and potentially assigning a reviewer for high
  priority items.

• The PR gets reviews, discussed and approvals as needed.

• The PR is merged by maintainers when it has been approved and comments addressed.

We currently aim to provide initial feedback/triaging within two business days. Please keep an eye on any labelling actions, as these will indicate priorities and status of your contribution.
We are very grateful for your contribution!

[bahner]

Incidentally, I believe this closes issue #186

[bahner]

@Jorropo when you take a look at this, then feel free to suggest actual writing to in-memory files. I just don't see any reason, but if there is then just holler.

[Jorropo]

I would like to support WASM too but I want to you know that you havn't added anything to the test suite.
If we don't test WASM in CI I could merge it but with expectations that it's not maintained, experimental and could break at any moment.

[Jorropo]

I don't think a new filewriter_wasm file is correct.
The STD emulates a unix filesystem on top of JS:
https://cs.opensource.google/go/go/+/master:src/os/file_unix.go;l=5?q=openFileNolog&ss=go%2Fgo

[Jorropo]

Btw, it's been since 1.18 that +build isn't used anymore.
I think you should upgrade your version of go. 😉 🙂

[bahner]

Hepp! Good feedback. A little busy here, but I'll follow up.