[bug] Reset lexer state when returning NEWLINE in webvtt_lex_newline()

The tests don't really check for this currently, but it's kind of required for simplicity (Otherwise, parse_timestamp() will see a timestam starting with \n or \r or \r\n, and not parse successfully)

set `self->tstate = L_START` when tstate == L_NEWLINE0 and returning NEWLINE token.

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Should also add these checks to each of the unit tests to ensure good behaviour in all cases.