Refactor `emitter` to better account for unicode chars when trimming

Change the way that underline positions are calculated by delaying using
the "visual" column position until the last possible moment, instead
using the "file"/byte position in the file, and then calculating visual
positioning as late as possible. This should make the underlines more
resilient to non-1-width unicode chars.

Unfortunately, as part of this change (which fixes some visual bugs)
comes with the loss of some eager tab codepoint handling, but the output
remains legible despite some minor regression on the "margin trimming"
logic.

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