tsb: small build improvements

[tmm1]

• tsb: normalize filenames consistently in add/removeScriptSnapshot
• tsb: avoid processing css imports

cc @jrieken

[AkiraYuki-04]

Add Return Consistency
Currently, the method returns the result of delete this._snapshots[filename], but if other deletions fail, this isn’t reflected. Instead, track and return the overall success:

[tmm1]

The return value is unused so I don't think it matters. The current implementation seems correct to me. The return value indicates whether a snapshot was present and deleted. The ancillary internal state is of no consequence to the caller.