Don't report 'tis, 'twould, etc.

[windymilla]

There is already a list of common initial apostrophe words, which is used in curly quote conversion, so it knows that is an apostrophe, not an opening straight quote.

I have repurposed that list to also say, don't report an apostrophe as an unexpected single quote if it's followed by one of those words.

Fixes #1917

This is the list of words (but with curly quotes instead of straight)

INIT_APOS_WORDS = (
    "'em",
    "'Tis",
    "'Tisn't",
    "'Tweren't",
    "'Twere",
    "'Twould",
    "'Twouldn't",
    "'Twas",
    "'Im",
    "'Twixt",
    "'Til",
    "'Scuse",
    "'Gainst",
    "'Twon't",
    "'Tain't",
)

It should work regardless of case.