add remove_if_then_else_expr

[Stefanuk12]

Similar to #202, a new rule called remove_if_then_else_expr would rewrite the following code from

local maxValue = if a > b then a else b

to

local maxValue
if a > b then
    maxValue = a
else
    maxValue = b
end

The official Luau syntax can be found here.

[Stefanuk12]

Satisfied by #207 and #208, whoops.