Add GraphValue and GraphMutation implementation

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 84 lines in your changes are missing coverage. Please review.

Project coverage is 17.80%. Comparing base (abbf4d6) to head (a189520).

Files	Patch %	Lines
Sources/OpenSwiftUI/Core/Graph/GraphHost.swift	0.00%	50 Missing :warning:
Sources/OpenSwiftUI/Core/Graph/GraphMutation.swift	0.00%	13 Missing :warning:
Sources/OpenSwiftUI/Core/Graph/GraphValue.swift	0.00%	10 Missing :warning:
Sources/OpenSwiftUI/Core/Graph/GraphInputs.swift	0.00%	9 Missing :warning:
.../Environment/Internal/TODO/CachedEnvironment.swift	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #43 +/- ## ========================================== - Coverage 18.22% 17.80% -0.42% ========================================== Files 158 160 +2 Lines 2947 3016 +69 ========================================== Hits 537 537 - Misses 2410 2479 +69 ```

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