IDE0044: Add readonly modifier (fields)

[h3xds1nz]

More info: https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0044

Should be done after #10284 / #10288 / #10354 to prevent merge conflicts

• IDE0044: Add readonly modifier (fields) - PresentationBuildTasks #10726
• IDE0044: Add readonly modifier (fields) - PresentationCore #10727
• IDE0044: Add readonly modifier (fields) - PresentationFramework #10728
• IDE0044: Add readonly modifier (fields) - WindowsBase #10730

[h3xds1nz]

Given the number of the files changed and the fact that this needs special care with mutable structs where I'm worried of false positives flagged by the analyzer, I will do these separately per-assembly.