[Impeller] libImpeller: Implement APIs for fetching glyph and line metrics.

[chinmaygarde]

This patch adds support in the typography subsystem of the public Impeller API such that users can implement text editing functionality.

• Line metrics for a fully laid out paragraph can be retrieved. The metrics contain information about offsets into the original code unit buffer used to create the paragraph. These offsets can be used to implement functionality that edits whole lines.
• Glyph information for a specific code unit offset, as well as a coordinate offset relative to the paragraph origin, can be obtained. This information can be used place decorations (like carets), select words surrounding the caret (a hit-test), and edit the source buffer to re-layout the paragraph.
• Word boundaries (as specified in Unicode Standard Annex 29) can be retrieved to select and modify paragraph subsets by character, word, and line at caret.

Like in Flutter, the code unit buffers are assumed to be arrays of UTF-16 bytes. I'd have preferred this to be UTF-8 because the initial paragraph construction is using UTF-8 bytes. Also, Skia internally seems to work with UTF-8 too but the interfaces are exposed using UTF-16 (presumably for users like Flutter that work with Dart strings that are UTF-16). Exposing additional APIs in txt::Paragraph to back this out seemed onerous. Instead, a UTF-16 assumption for all APIs that retrieve metrics is made (and documented). It stands to reason that paragraphs should be constructable using UTF-16 buffers in the public API too. I'll add that in a subsequent patch as that has little to do with metrics.

Fixes #165509

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autosubmit label was removed for flutter/flutter/165701, because - The status or check suite Linux linux_unopt has failed. Please fix the issues identified (or deflake) before re-applying this label.

[auto-submit]

autosubmit label was removed for flutter/flutter/165701, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.