Support color `hwb()` function and store HSL colors natively without converting to RGB

[jwmcglynn]

Related to #6

Implements support for HSLA color representation and updates the Color class to handle deferred conversion from HSLA to RGBA. Additionally, updates and adds unit tests to validate these changes.

• Color Class Enhancements

  • Adds an HSLA struct to represent HSLA color values, including hue, saturation, lightness, and alpha components.
  • Introduces hasHSLA() and hsla() methods to check for HSLA color type and retrieve HSLA values, respectively.
  • Updates the resolve() method to include logic for converting HSLA to RGBA, though the actual conversion logic is marked as a placeholder.
  • Modifies the output stream operator to support HSLA color output.

• Unit Test Updates

  • Adds a new test case in Color_tests.cc to verify deferred conversion functionality for HSLA colors, ensuring that HSLA colors are correctly identified and their components are accessible.

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For more details, open the Copilot Workspace session.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.66667% with 26 lines in your changes missing coverage. Please review.

Project coverage is 88.87%. Comparing base (af5964f) to head (4e3e2a8).

Files	Patch %	Lines
donner/css/parser/ColorParser.cc	67.44%	10 Missing and 4 partials :warning:
donner/css/Color.cc	79.59%	9 Missing and 1 partial :warning:
donner/css/Color.h	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #166 +/- ## ======================================= Coverage 88.87% 88.87% ======================================= Files 249 249 Lines 19931 20013 +82 Branches 2062 2064 +2 ======================================= + Hits 17713 17786 +73 - Misses 1613 1615 +2 - Partials 605 612 +7 ``` | [Flag](https://app.codecov.io/gh/jwmcglynn/donner/pull/166/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+McGlynn) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jwmcglynn/donner/pull/166/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+McGlynn) | `88.86% <82.66%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+McGlynn#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jwmcglynn]

Implements the hwb() function parsing in ColorParser and adds corresponding unit tests.

• Adds parsing logic for the hwb() function within ColorParser::parseColor(), handling conversion from HWB to RGBA color space, including optional alpha value handling.
• Introduces unit tests in ColorParser_tests.cc for various HWB color values and their conversion to RGBA, verifying correct parsing and conversion, including tests with optional alpha channel.

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For more details, open the Copilot Workspace session.