eval: Add luaeval function

No tests yet, no documentation update, no :lua* stuff, no vim module.

converter.c should also work with typval_T, not Object.

Known problem: luaeval("1", {}) results in

    PANIC: unprotected error in call to Lua API (attempt to index a nil value)

Ref #3823

src/nvim/eval.lua

@@ -193,6 +193,7 @@ return {
     localtime={},
     log={args=1, func="float_op_wrapper", data="&log"},
     log10={args=1, func="float_op_wrapper", data="&log10"},
+    luaeval={args={1, 2}},
     map={args=2},
     maparg={args={1, 4}},
     mapcheck={args={1, 3}},