Add skip_typescript to add string enums in web-sys

[RunDevelopment]

Temporary solution for #4163.

I changed the webidl generator code and added a test case to see whether the fix works.

However, running the command from the web-sys README fails on my system, so I cannot re-generate the rs files.

PS C:\Users\micha\Git\wasm-bindgen> cargo run --release --package wasm-bindgen-webidl -- webidls src/features ./Cargo.toml
    Finished `release` profile [optimized] target(s) in 0.30s
     Running `target\release\wasm-bindgen-webidl.exe webidls src/features ./Cargo.toml`
Error: reading webidls directory

Caused by:
    Das System kann den angegebenen Pfad nicht finden. (os error 3)
error: process didn't exit successfully: `target\release\wasm-bindgen-webidl.exe webidls src/features ./Cargo.toml` (exit code: 1)

[RunDevelopment]

And thank you for running the command for me.

[RunDevelopment]

We decided against this solution: #4163 (comment)