Minor optimization for `MergedStyle.SetStyle`

[MartyIX]

Description of Change

This PR just removes a few allocations. There is no big idea.

Benchmark

cd src/Core/tests/Benchmarks
dotnet build -c Release --framework net8.0 && dotnet run --framework net8.0 -c Release -- --runtimes net8.0 --filter *StyleBenchmarker*

MAIN

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Gen2	Allocated
MergedStyle	66.18 us	2.060 us	5.977 us	63.38 us	4.5166	4.3945	0.7324	28.25 KB

PR

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Gen2	Allocated
MergedStyle	64.74 us	1.820 us	5.367 us	62.34 us	4.5166	4.3945	0.7324	28.25 KB

The improvement is not really big but I find it better than nothing.

Additional findings

1. It appears that MergedStyle.ReRegisterImplicitStyles is not in use (at least not in this repo).
2. CA1859 is reported on line https://github.com/dotnet/maui/blob/d7dac41200a932ea748569a28b6f26e7e3df69db/src/Controls/src/Core/MergedStyle.cs#L15 and it looks like it can be trivially changed and nothing wrong would happen.

Issues Fixed

Fixes #21539

[rmarinho]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 3 pipeline(s).

[MartyIX]

@StephaneDelcroix Does this PR make sense to you please?

[MartyIX]

@jonathanpeppers Does the proposed changes make sense to you please?