TODO: Add support for long strings

[CFiggers]

Currently the lexer recognizes only "regular" (short?) strings (i.e., those wrapped with " regular quotation marks). Janet supports long strings, which currently is not recognized by the lexer.

This causes the janet:cursorInString flag to be false even when the cursor is inside of a long string.

This causes structural editing features to misbehave when the cursor is inside a long string.

Also, auto indentation inside long strings is unpredictable.