Handle local-variable hiding in local functions

[meziantou]

Summary

• Report MA0084 when a local function declares a variable that hides a local from an enclosing method
• Skip the diagnostic for static local functions and nested cases that cannot access the outer local
• Add analyzer coverage for the new local-function scenarios and update the rule docs

Testing

• Added unit tests for local, static local, and nested local function hiding cases
• Not run (not requested)