Improve tags (#206)

- Add stricter tests: tags cannot be benchmark names to avoid accidental selection
- Each benchmark needs to have at least one of "js" or "wasm" tag
- "Deafult" tag is now lower-case
- Add "bigint" to bigint tests
- Rename "regexp" to "regexp-octane" to avoid clash with the tag of the same name

Author: camillobruni