feat: add Ch and Ex helper functions for character-based CSS units

[Laaaaksh]

Fixes Issue - #170

• Add Ch(value float64) function for character units (ch)
• Add Ex(value float64) function for x-height units (ex)
• Include comprehensive tests for both functions
• Follow existing patterns for float64 unit functions

[Copilot]

Pull Request Overview

This PR adds support for two CSS character-based units by introducing helper functions Ch() and Ex() for generating character unit (ch) and x-height unit (ex) CSS values respectively.

Key Changes:

• Added Ch() function to generate CSS character unit values with float64 input
• Added Ex() function to generate CSS x-height unit values with float64 input
• Added comprehensive test coverage for both functions following existing test patterns

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
styles/utils.go	Implements Ch() and Ex() helper functions for character-based CSS units
styles/utils_test.go	Adds test cases for Ch() and Ex() functions covering various input scenarios

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[chasefleming]

lgtm, thanks!