feature: support generics in function type format

[TheFireBlast]

Currently, the only way to document generic functions is to use @generic. and it's not possible to use them in overloads or fields.

The following examples are invalid:

---@generic T
---@param arg1 string
---@param arg2 T
---@return T
---@overload fun(arg2: T) <- Undefined type or alias `T`.
local function foo(arg1, arg2)
 return arg2 or arg1
end

---@class Foo
---@field bar fun<T>(arg:T): T <- `(` expected.
local Foo = {}

[stylpe]

#723 seems like it already covers this?

[PoolloverNathan]

+1, porting Haskell's ReadP to Lua and I need a fun<U>(fun(T): P<U>): P<U>.

[sharpchen]

T should also be visible inside function body

---@generic T
---@param list? T[]
local function foo(list)
 ---@cast list T[] -- warning: Undefined type or alias `T`.
end