Add convert_function_to_assignment rule

[jeparlefrancais]

Closes #11

Add a new rule to convert function declaration statements into regular assignments.

local var = {}

function var.func() end
-- turns into:
var.func = function() end

• add entry to the changelog

[github-actions]

Coverage after merging convert-function-to-assignment into main will be

87.29%

Coverage Report for Changed Files

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src/rules
global_function_to_assign.rs	100%	100%	100%	100%
mod.rs	90.03%	100%	83.78%	90.82%	140–142, 161–163, 178, 199–201, 203–204, 249–251, 367, 407–409, 436, 443–446, 460