* implement a fix for tags being found in code blocks

still needs polish, consider this a rough draft

* refactor to reuse a local function

uses CheckWrappedInCode to check that there are no code blocks that surround the tag being parsed

* Add more test coverage of MessageHelper.ParseTags

* reset indexes for @ here mention

* add a test case to catch error fixed from prev commit

* wip commit of most test cases working

* fix the Enclosed in block util method

* code cleanup

* lint whitespace

* lint brackets for single line if blocks

* move messagehelpertests to the new unit test dir

* expose internals to the unit test project

this seems to have been breaking the build, since CI would build the merged branch, where rest wasn't exposed to the unit tests

Previously it was implemented under Discord.Net.Rest, which seems
inconsistent and unnecessary.

This also allows Commands docstrings to reference EmbedBuilder, since
Commands only has a dependency on Core.