fix: add surrogate pair support for font substitution and text encoding

[romen2232]

Overview

This PR adds support for handling surrogate pairs (characters outside the Basic Multilingual Plane) in font substitution and text encoding. This improves the library's ability to handle modern Unicode characters correctly.

Context

In our project, we encountered issues when rendering mathematical symbols and special characters. For example, when attempting to display mathematical italic characters like 𝑥, the text rendering would break because these characters are represented as surrogate pairs in Unicode. The existing implementation didn't properly handle these cases, leading to incorrect rendering (each character in the pair was rendered as a different character).

Changes

Font Substitution Engine

• Updated the font substitution engine to properly handle surrogate pairs by using codePointAt() instead of character-by-character iteration
• Added proper character length calculation for surrogate pairs (2 positions) vs regular characters (1 position)
• Improved run splitting logic to maintain correct character boundaries

AFM Font Encoding

• Modified the encodeText method in AFMFont to handle surrogate pairs correctly
• Updated glyphsForString to use codePointAt() for proper Unicode character handling
• Improved character iteration logic to account for surrogate pair lengths

Testing

Added comprehensive test coverage for surrogate pair handling:

• Basic surrogate pair handling in text
• Multiple surrogate pairs in sequence
• Surrogate pairs interspersed with regular text
• String conversion utilities for surrogate pairs
• Mixed regular and surrogate pair code points

Testing

The changes include extensive test coverage in:

• packages/textkit/tests/engines/fontSubstitution.test.ts
• packages/textkit/tests/utils/stringFromCodePoints.test.ts

Test cases cover various scenarios, including:

• Single emoji characters
• Multiple emojis in sequence
• Mixed content with regular text and emojis
• Edge cases with different Unicode ranges

Impact

This change improves the library's ability to handle modern Unicode text, particularly:

• Emoji characters
• Non-BMP Unicode characters
• Mixed content with both regular text and special characters

Breaking Changes

None. This is a backward-compatible enhancement that improves existing functionality.

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Latest commit: 16aa4b4

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[romenmedina-aircury]

Relevant issue
#3127