Enhance explicit interface implementation completion provider

[Rekkonnect]

Fixes #75435
Fixes #72224
Fixes #70458

Implementation details

The ExplicitInterfaceMemberCompletionProvider is now up to par with the OverrideCompletionProvider, with some abstractions to share between the two like the ItemGetter. The characters [ and < no longer commit the selected completion item since the completion provider now fills the implementation body and the signature completely.

The AbstractMemberInsertingCompletionProvider itself also became faster by doing less work when replacing the completed text in the original document via annotations. This behavioral adjustment was also necessary because of problems that arose while changing the ExplicitInterfaceMemberCompletionProvider to be an AbstractMemberInsertingCompletionProvider.

Another fixed bug is the explicit interface implementation generator for abstract conversion operators, which were not handled in the "Implement interface explicitly" code fixes. This was resolved for both the fixes and the completion provider.

When the user is typing out an explicit interface declaration member, the provider ignores the potential difference in the return type. This is justified as the user could easily mistake the return type, and so we do not want to filter out the actual candidates that can be offered. Besides, in most cases the recommended members are too few to need this filtering. If the user types out a return type and then we complete a member that has a different return type, the result includes the actual signature of the implemented member, discarding the old return type.

The spell fix suggestions for explicit interface implementation members were removed because of the complexity of handling the generated code. The justification is that it's easier for the user to manually type the member out because of the improvements to the completion provider.

The symbol generators still ignore the attributes on the parameters or the symbols themselves of explicit interface implementations, despite being legal to use in that context.

Tests

• Added a bunch of tests showing the new behavior of the completion provider
• Changed existing tests to reflect the new behavior
• Changed the spell fix suggestion tests for explicit interface implementation members to expect no fixes
• Added a test for C# completion offered for explicit static interface method declaration generates code with several issues #70458
• Added tests showing the behavior on nested interfaces (outer types being interfaces or other kinds)

[CyrusNajmabadi]

Thanks!