fix: do not crash when comparing type defined with 'function'

Using the plain 'function' type needs to define 'min_arity'.

spec/assignment/to_function_spec.lua

@@ -0,0 +1,12 @@
+local util = require("spec.util")
+
+describe("assignment to function", function()
+   it("does not crash when using plain function definitions", util.check([[
+      local my_load: function(string, ? string, ? string, ? table): (function, string)
+
+      local function run_file()
+         local chunk: function(any):(any)
+         chunk = my_load("")
+      end
+   ]]))
+end)

tl.lua

@@ -2577,6 +2577,8 @@ do
       else
          typ.args = new_tuple(ps, i, { new_type(ps, i, "any") }, true)
          typ.rets = new_tuple(ps, i, { new_type(ps, i, "any") }, true)
+         typ.is_method = false
+         typ.min_arity = 0
       end
       return i, typ
    end

tl.tl

@@ -2577,6 +2577,8 @@ local function parse_function_type(ps: ParseState, i: integer): integer, Functio
    else
       typ.args = new_tuple(ps, i, { new_type(ps, i, "any") }, true)
       typ.rets = new_tuple(ps, i, { new_type(ps, i, "any") }, true)
+      typ.is_method = false
+      typ.min_arity = 0
    end
    return i, typ
 end