(getSelection, focus, setParser, cursorCoords)

This is already going off into way-out hackiness again, but
textwrapping is useful, so I'll see if I can get it to work.

whitespace is now no longer 'pulled into' the token next to it,
but put in class-less spans. parsers can return null to indicate
they don't want to assign a class to the consumed text. the js
parser now also does this for operators and punctuation, so
that the amount of joinable spans is much greater

this makes it less likely that a held cursor-movement key fires twice
before we read its effect, which could make it hit the end of the
textarea text, creating weird jumping effects

data on token styles and selection is neatly accumulated in the Line
data structure, and then rendered to HTML in one shot.

also introduces a Line class to organize code at least a little

this keeps track of whether the selection and the textarea
have to be updated, so that this logic isn't scattered
through the code, and to prevent superfluous updates