Expose way to add custom JsonConverters to support external Roslyn OOP services

Prioritize NavigateTo searches to care most about the current actives docs/project.

README update

Update Build.proj

* Precedence: explicit selection -> provider preference -> last selection -> first/default value.
* Remove logic to bail out on retrigger

Merge dev15.9.x to master

Make PinnedSolutionInfo type sealed

Merge dev15.9.x to master

Merge dev15.8.x to dev15.9.x

Merge dev15.9.x to master

* Add AB tests for code cleanup
revert the text change of Code_cleanup_is_not_configured, as the new text is not translated
Only SettingIsAlreadyUpdatedByExperiment to ture for Group B; always show gold bar if user setting is changed in AB test

Add a namespace-glyph when spell-checking+add-using.  

Typos in "SHA1" spelling

Use explicit date format provider to avoid tests breaking in different locales

Make sure TLS 1.2 is used to fetch from https://dot.net

Make sure TLS 1.2 is used to fetch from https://dot.net

Fix rename with F# projects referencing C# projects

* remove random last token from engine.

* put the support back to what it used to, but in different way.

* PR feedback

* added comments on the typing rule.

* PR feedbacks

https://github.com/dotnet/announcements/issues/77 for the change and
where I am stealing this change from.

Disable icacls use on Helix

Remove PR template

We are no longer in ask mode hence no reason to have this around.

The icacls command is failing on Helix hence disabling these tests there
for now

Related to #28836

Use FilePath in VisualStudioWorkspaceImpl.AddDocumentToProjectItems

If we are doing a rename from a C#/VB symbol, and an F# project depends
on that project, we might end up walking into non-C#/VB projects and
we will crash. This prevents the crash.

There were two ways I could fix this bug: one would be to update
GetDocumentsAffectedByRename to exclude them up front. The other option
is to deal with those at the call sites when we fail to find a service.
I'm electing to do the later as that allows light-up by the appropriate
language services if they wanted to do that down the road.

AddNonSubmissionDependentProjectsAsync is designed to return projects
that depend on a given symbol. It indirectly calls HasReferenceTo
which in the metadata paths will only return projects that have
Compilation support. Thus, it seems safe to exclude the projects
entirely.

Fixes dotnet/roslyn#25655.

Bump our .vsixmanifests to say we support installation on Dev16