Make RegexParseException and RegexParseError public, rename enum fields (#40902)

Make RegexParseException and RegexParseError public, rename enum fields (#40902)

* RegexParseException, RegexParseError public, rename enum fields

* Add Unknown field to RegexParseError

* Add definitions to ref file

* Satisfy NET48 tests

* Some nits to resolve review comments

* Add default message and Unknown message

* Added doc comments

* Fix nit in auto-layout change of csproj file

* Simplify message for RegexParseException ctor overload

* Remove PlatformDetection.IsNetFramework in RegexParserTests.cs

* Rename resource names to match the enum names

* Use partial class and methods to separate netfx and core

* Add offset tests

* Apply suggestions from code review

Nits

* Add remaining test offsets. Verify all offsets.

* Nit and TFM

* Update src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netcoreapp.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netfx.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Update src/libraries/System.Text.RegularExpressions/tests/RegexParserTests.netfx.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Code review: properly test for deserialized type of exn

* Code review: fix doc comments

* Code review: fix doc comments

* Code review: offset gt zero

* Code review: add some tests, remove redundant whiteline

* Code review: add doc comment as suggested

* Code review: fix doc comments

* ref edits with tool

* Code review: some nits

* Remove using

* Rename unknown

* Incorrect edit in AssertExtensions

* Cherry-pick #d4090296

* Fix naming, ref file, resource strings

Co-authored-by: Dan Moseley <danmose@microsoft.com>
Co-authored-by: Prashanth Govindarajan <prgovi@microsoft.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>