fix(creds): allow periods in TF_TOKEN_... credentials vars

[brandonc]

After re-reviewing terraform-credentials-env, I made a mistake in not allowing period characters in TF_TOKEN_... variable names, nor allowing case-insensitive variable names. I now see that being liberal with variable names by using svchost.ForComparison on the variable name itself can allow for all these variations without a bunch of guessing code. I should have just lifted @apparentlymart 's code to begin with!

The only addition I made was to encode hyphens as double underscores. My reasoning about why this is OK is captured in the comment:

We accept double underscores in place of hyphens because hyphens are not valid identifiers in most shells and are therefore hard to set. This is unambiguous with replacing single underscores below because hyphens are not allowed at the beginning or end of a label and therefore odd numbers of underscores will not appear together in a valid variable name.

Can you double check my logic there?

I also revised the docs accordingly @laurapacilio

[apparentlymart]

This seems reasonable to me!

I left an inline note about a dangling comment that I expect was probably just a TODO you forgot to remove after you TODID it 😀

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