Feature: resolve x/y in text block to string char index

[lojjic]

This is a necessary feature for eventually enabling text selection/highlighting/editing.

We need the ability to raycast a position in a rendered text block, and for that position determine the closest character boundary to it, returning an index in the original text string.

It's easy enough to resolve the glyph index, by searching through the glyph rects array, but there isn't a clean way to resolve the glyph index back to string char index. These aren't always 1-to-1 due to ligature substitution etc. This may require modifying Typr.js to persist char indexes with the glyphs.

[lojjic]

This has been implemented in selectionUtils.js#getCaretAtPoint which resolves an x/y to the nearest caret position. The resulting caret object holds a charIndex property.