feature: support generics in function type format

Nobody has claimed this yet.

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:

```lua
---@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
```

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

No source files or tests are named. Start by locating the parsing and type-resolution paths for @generic, @overload, and function types, then compare them with the two Lua examples. Done means generic type parameters resolve inside overload signatures and field function types without the reported diagnostics.

Written by the indexing model from the issue text.