Use 'webvtt_parse_cuesetting' for 'vertical' cuesettings

An interesting note that isn't addressed in this PR, but will be coming up in a subsequent one:

The 'BadWhitespaceBefore...'/'BadWhitespaceAfter...' tests are interesting, because the parser currently behaves differently depending on whether whitespace appears before ':' or after ':'. In the former case, the cuesetting is skipped past and ignored, while in the latter case, the setting is still used.

It doesn't really make sense for this to be separate behaviour, and I think the simplest solution will actually be to remove the UNEXPECTED_WHITESPACE errors, and simply treat unexpected whitespace errors as bad/invalid cuesettings.

However, we might still wish to provide a warning that this unexpected whitespace has been used, in which case we could ignore the cuesetting regardless of where the whitespace occurs, and still return a meaingful error to the application.

I'm not sure what will make sense just yet.