Fixes #64159 - JsonSerializerContext source generation fails with keyword identifiers...

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>