Delete a bunch of code on Roslyn side, which is no longer necessary as we don't own the column anymore.
I have also cleanup up some code in this space for newly added ContainingType and ContainingMember columns.

* Both columns added

* Genericize custom column names

* More changes to genericize

* Cleanup

* Respond to feedback

* WIP

* Change tuple to properties

* Restrict to just containing types

* Cleanup

* cleanup

* Refactor

* Change method for getting Containing Type and Member

* refactor GetAdditionalProperties

* Fix merge conflicts

* respond to feedback

* respond to feedback

* Change to struct

* Add type for AbstractCustomColumnDefinitionWithMultipleValues

* Cleanup

* Change AdditionalProperty to FindUsagesProperty

* respond to feedback

* respond to feedback

* Hide columns in F#

* respond to feedback

* fix merge conflict

* Change from hiding columns to removing columns for non supported langs

* Add additional implementations

* formatting

through IDynamicFileInfoProvider and etc, external component such as razor should be able to provide us dynamic file (such as generated c# content for cshtml file) on the fly and let us know update on the content.

renamed Column to Character since Column should be only used when it accounts "tab". and count "tab" as tab size rather than just 1 char.

moved FAR to work on top of new set of APIs. it should now show things on html correctly

added support for code lens. but found 1 issue. without changing VS side, I can't fix filename. and without fixing filename, navigation won't work properly

Implementation of the custom column support has been kept extensible, so that each reference item can have customized key-values info, where key corresponds to the name of the custom dynamic column and values are set of one or more values the table entry. Columns become dynamically visible if at least one row in the table has a value for it.

Fixes #24877

We don't need the split anymore now that we're only supporting Dev15.

Unfortunately this required the introduction of a NuGet package that
has a namespace "Workspace" to our VS project, which meant I had to
disambiguate Workspace wherever it's used in that layer. I considered
doing an extern alias instead to disambiguate, but applying that to a
ProjectReference isn't supported. That's tracked by NuGet/Home#4989.

This reverts commit 8262112e, reversing
changes made to 837ca214.

Also, update ProjectDisplayName to do what the docs say it does for Venus files.

Partially addresses #18382.