Razor C# light bulb (code actions) support

[NTaylorMullen]

This issue serves as a tracking issue to capture the initial list of Razor C# light bulbs that we're looking to deliver in the .NET 5 timeframe. This list is an attempt to capture the mainline light bulbs that are used in C# but not meant to be an exhaustive list. Note that it's very likely that this list will grow and shrink as time progresses.

In scope

1. Add usings
2. Remove and sort usings
3. Refactor to code behind
4. Fully qualify type name
5. Extract to local
6. Extract to method
7. Extract to local function
8. Change signature
9. Implement interface
10. Implement abstract class
11. Simplify expression
12. Remove unused code
13. Rename

On the cut line

1. Generate XYZ for private classes
   1. Field
   2. Read-only field
   3. Property
   4. Read-only property
   5. Read-only field
   6. Parameter