Formatting and parsing changes to JsonString (dotnet/corefx#41940)

Formatting and parsing changes to JsonString (dotnet/corefx#41940)

* Pull duplicated code into JsonWriterHelper

* Add helpers for parsing and formatting ISO dates

* Update tests to expect new format

* Use new helpers in JsonString methods

* Rename methods

* Check for valid parse length

* Use AsSpan extension to work around missing implicit operators on netfx

* More netfx workarounds

* Use constant for stackalloc length

* Add back temp span to avoid writing too much when trimming

* Move IsValidDateTimeOffsetParseLength to JsonHelpers

* Always use StackallocThreshold instead of calling GetUtf8ByteCount

* Remove redundant length checks done by IsValidDateTimeOffsetParseLength

* Adapt test pattern from Utf8JsonWriterTests

* Revert "Always use StackallocThreshold instead of calling GetUtf8ByteCount"

This reverts commit dotnet/corefx@0d29b646166138413f311a3ccf6284a72024b439.

Commit migrated from https://github.com/dotnet/corefx/commit/21fd4d2a70447064aacb6536278f6e8cc1c9e2d5