Incorrect CSS namespace in hardcoded frontend classes

[RichDom2185]

See source-academy/js-slang#1552.

Some frontend classes still use 'bp4', which causes some regressions following our update to Blueprint v5. The original PR #2718 did update the CSS namespace in SCSS files, but not TS files.

Just another reason we should move out of the API, as detailed in #2722.