Add method to retrieve information about what characters in the original string were replaced by each glyph.

[opengraphica]

Add a stringToGlyphMapping API to font.mjs to retrieve information about what characters in the original string were replaced by each glyph.

Description

There exists stringToGlyphIndexes and stringToGlyphs APIs, which convert a string to an array of glyphs. Unfortunately, information about which character is transformed into each glyph is lost when using these methods.

This PR adds a stringToGlyphMapping method which returns a list of glyphs, as well as character indices in the original string that was provided to be able to track which characters were replaced by each glyph.

stringToGlyphMapping(s: string): Array<{ glyph: Glyph, replaced: number[] }>;

Example Input: "fla"
Example Output: [ { glyph: Glyph, replaced: [0, 1] }, { glyph: Glyph, replaced: [2] } ]

Motivation and Context

A text editor needs to know which characters the user typed are converted to each glyph, otherwise it is impossible to implement text highlighting and cursor placement.

For example, if a text document has ["f", "l"] and that is transformed into a single glyph for display ["fl"]. Pressing backspace should operate on the original array ["f", "l"] and modify the text document into ["f"]. It can't operate on the "fl" glyph.

How Has This Been Tested?

Beyond the unit tests, checked in editor code https://github.com/opengraphica/opengraphica. This adds a new API that does not touch existing APIs, so it should not negatively impact any other part of the codebase.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the Contribute README section.