Fixes https://github.com/flutter/flutter/issues/131435, https://github.com/flutter/flutter/issues/104594, https://github.com/flutter/flutter/issues/43400

Currently the method we use for text span hit testing `TextPainter.getPositionForOffset` always returns the closest `TextPosition`, even when the given offset is far away from the text.

The new TextPaintes method tells you the layout bounds `(width =  letterspacing / 2 + x_advance + letterspacing / 2, height = font ascent + font descent)` of a character, the PR changes the hit testing implementation such that a TextSpan is only considered hit if the point-down event landed in one of its character's layout bounds.

Potential issues:

In theory since the text is baseline aligned, we should use the max ascent and max descent of each character to calculate the height of the text span's hit-test region, in case some characters in the span have to fall back to a different font, but that will be slower and it typically doesn't make a huge difference.
This is a breaking change.