`generate` Gradle task not idempotent

[severn-everett]

Reproduction Steps:

1. Run the Gradle task generate
2. Re-run the Gradle task generate

Expected Behavior:

generate task produces the same HTML classes.

Actual Behavior:

Additional content gets produced for gen-tag-groups.kt and gen-tag-unions.kt

[severn-everett]

I've got a PR ready for this; could it be looked at?

[e5l]

Hey @severn-everett, thanks for the PR, let me check