Type inferencing of filter style functions

Given:

---@generic T
---@param f fun(a: T)
---@param t table<any, T>
---@return T[] (table)
local function tbl_filter(f, t)
  return t
end

---@type string[]
local s = {'a', 'b', 'c'}

local r1 = tbl_filter(function(a) end, s)

r1 is inferred as unknown[] because a is unknown despite s being known. However, could we infer a from s and force the typechecker to insist a is a string?

If I remove the type from a:

---@generic T
---@param f fun(a)
---@param t table<any, T>
---@return T[] (table)
local function tbl_filter2(f, t)
  return t
end

local r2 = tbl_filter2(function(_) end, s)

r2 is now inferred as string[] because only s is used to infer the type.

首先复现 issue 中的两个 tbl_filter 示例，并比较推断出的回调参数类型和结果类型。跟踪 language server 的 generic-function 和 callback type-inference 入口；完成的标准是：第一个回调参数从 s 推断为 string，r1 为 string[]，并且不兼容的回调类型会被拒绝。