Fixes #64159 - JsonSerializerContext source generation fails with keyword identifiers (#66876)
* Fixes #64159 - initial implementation
* Tidy up, and add test for ignored reserved-keyword-named property
* PR feedback - use same approach as logging generator
* PR feedback
* Update src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs
Co-authored-by: Adeel Mujahid <3840695+am11@users.noreply.github.com>
* PR feedback - rename ExactNameSpecifiedInSourceCode
* PR feedback - use extracted (and renamed) local variable
* Remove commented code
* Added `IsVerbatimName` as extension method
* Support fields with verbatim names (@)
* Use alternative method for checking for verbatim names
* Uses `SyntaxFacts` to determine if escaping is needed
* Remove extension method
* Modified source generator test to include a verbatim field
* Minor typo
Co-authored-by: Adeel Mujahid <3840695+am11@users.noreply.github.com>