Two Rule Suggestions: multiple returns in function calls / table definitions and string concatenation

### multiple returns in function calls / table definitions

```lua
local a = { "one", "two", "three" }
local b = { unpack(a), "four" }
local c = { "four", unpack(a) }
print(#b) -- 2
print(#c) -- 4
```
`unpack`'s behavior is quite unintuitive; when the output is not in the last position of a table definition, only the first element is actually included in the new table. (In fact, this not only applies to `unpack`, but to every function that returns multiple values – only happens most often with `unpack` that will people will notice, I assume.)

see explainer in the docs: https://www.lua.org/manual/5.3/manual.html#3.4

I think a diagnostic that reminds one of such cases would be very useful, since it's a common gotcha in lua.

## string concatenation

```lua
print("foo" + "bar")
```
Common mistake to not use `..` for string concatenation. Even though I am used to lua, it still happens to me all the time after coming back from writing in a different language. A diagnostic here would be useful. (And I am actually a bit surprised that there isn't one yet.)

Start with the Lua 5.3 manual section linked in the issue, especially the multiple-return rules illustrated by the table examples. Then locate the language-server diagnostic entry points for multiple-return expressions in table definitions and for using + with strings. Done means both examples produce useful diagnostics without incorrectly flagging valid Lua code.

Written by the indexing model from the issue text.