[LS] Add support for code actions

[SupunS]

Issue To Be Solved

Code actions provide a convenient way for the users to fix issues in their code.

Suggest A Solution

• Compiler diagnostics (semantic errors) are generally used to figure out and provide code actions
• Some of the very useful code-actions are:
  • Create variables (when an undefined variable is referred)
  • Change access modifier
  • Add a type cast or conversion function call when there are incompatible types
  • Change variable/parameter/return/field type

[darkdrag00nv2]

Create variables (when an undefined variable is referred)

This seems to be done already.