Add support for mixin composition

[ArizmendiWan]

Allows mixins to reference other mixins via a reserved mixin key. Resolves #39.

A mixin definition can now include a list of other mixin names to compose:

build:
  debug:
    cmake-args: ["-DCMAKE_BUILD_TYPE=Debug"]
  asan:
    cmake-args: ["-DSANITIZE_ADDRESS=ON"]
  debug-asan:
    mixin: ["debug", "asan"]
    cmake-args: ["-DEXTRA_FLAG=1"]

Using --mixin debug-asan resolves all referenced mixins recursively before applying the mixin's own arguments. Precedence matches existing CLI --mixin behavior:

• Lists are prepended (later mixin values come first, matching _update_args)
• Scalars between references use first-write-wins (matching how _update_args skips already-set scalars)
• Own args always override referenced values

Also adds validation and tests for:

• missing references
• composition cycles
• invalid mixin definitions
• parser-level composed mixin application and error paths

Existing mixin files without a mixin key are unaffected.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.09%. Comparing base (ebedd27) to head (7822bef).

Files with missing lines	Patch %	Lines
colcon_mixin/mixin/__init__.py	87.09%	2 Missing and 2 partials ⚠️
colcon_mixin/mixin/mixin_argument.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   15.30%   19.09%   +3.79%     
==========================================
  Files          11       11              
  Lines         562      597      +35     
  Branches       94      106      +12     
==========================================
+ Hits           86      114      +28     
- Misses        474      479       +5     
- Partials        2        4       +2     

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[ArizmendiWan]

Added a follow up commit to tighten mixin composition validation.