Support Anonymous Function Assignments

[YetAnotherClown]

Resolves #139

Supports Anonymous Function Assignments, such as the following

--- @within Class
class.foo = function(x: number): number end

--- @within Class
local freeFunction = function(x: number): number end

--- @within Class
bar = function(x: number): number end

When multiple variables or expressions are present, the Extractor will output the following Diagnostic:

error: Assignments cannot have more than one variable
  ┌─ test-input/failing/anonymous_function_assignment.lua:6:1
  │  
6 │ ╭ --- @within Class
7 │ │ --- Multiple variables should fail
  │ ╰──────────────────────────────────^ Assignments cannot have more than one variable
8 │   class.foo, class.bar = function(x: number): number end, function(y: string): string end
  │              --------- Additional variable here