Add Utf8JsonWriter along with unit tests (dotnet/corefx#34425)

Add Utf8JsonWriter along with unit tests (dotnet/corefx#34425)

* Move JsonReader related files to the Reader directory.

* Update S.T.Json ref to include new JsonWriter APIs.

* Port initial JsonWriter files from corefxlab.

* Auto-generate System.Text.Json ref to update the ordering for
consistency.

* Fixed some TODOs, delete dead code, and formatting cleanup.

* Make use of self-descriptive constants where possible.

* Fix leftover TODOs and update throw helper/exception messages.

* Add xml comments/docs to the public surface area.

* Add JsonWriter unit tests.

* Change GetCurrentState back to a property and explicitly Flush on the
callers behalf instead of throwing.

* Save current depth as part of state and update tests.

* Fix constant names, account for quotes in bytes needed, and add fixed buffer writer tests.

* Fix inconsistent use of braces by adding them for single line ifs

* Update parameter name to exclude encoding.

* Remove JsonWriterException and use InvalidOperationException instead.

* Use Rune and Utf8Formatter/TryFormat in more places and remove
UnicodeScalar.

* Fix nits, typos, and reorder field assignment and method calls.

* Pass spans by in (or by value) instead of by ref.

* Update comments and remove unnecessary test.

* Remove some aggressive inlining and pass spans by value rather than in

* Update comments, dont compute bytes needed, and use if instead of loop before advancing.

* Reduce code bloat by removing duplciate calls to ValidateX.

* Add details on how .NET types are formatted to comments.

* Reduce code duplication when writing values.

* Change the StandardFormat used for DateTime(Offset) to 'O'

* Refactor calculating the maximum escaped length to a helper.

* Remove unnecessary checks and rename locals to be more descriptive.

* Rename suppressEscaping to escape and flip default from false to true.

* Comment cleanup, add debug.asserts, and move transcoding helpers to a
separate file.

* Increase the deicmal max size to account for sign and add tests.

* Rename ROS<byte> property name and value params to include utf8 in the
name.

* Remove redundant code where idx is set to 0 unnecessarily.

* Remove dead code (dont escape forward slash) and make tests culture
invariant.

Commit migrated from https://github.com/dotnet/corefx/commit/f84927d4a85444e63ede2da38a49eff7116790db