Speed up source map emit for large single-line object literals

[Copilot]

Source map emit repeatedly recalculated the generated UTF-16 column from the start of the current line. Files containing large single-line object literals therefore incurred quadratic scanning costs.

• Writer optimization

  • Cache the last measured output position and UTF-16 column.
  • Scan only text appended since the previous lookup.
  • Reset cached state when output advances to a new line.

• Coverage

  • Validate incremental UTF-16 accounting with astral characters.
  • Validate cache resets across raw newlines and indented lines.

• Fixes sourceMap emit is disproportionately slow for files containing one very large object literal #64134

[Jake Bailey (jakebailey)]

Interesting, I think we have this same optimization somewhere else? I had thought for this specifically, actually.