Syntax warning / error not flagged as such.

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Diagnostics/Syntax Checking

### Expected Behaviour

A warning or an error that the line in question is not complete, regardless of how the parser it's run in processes it:
```lua
local varA, varB functionCalledHere()
```

### Actual Behaviour

No warning or error was given, resulting in a 20 minute hunt for a `nil` in one of the variables that shouldn't exist. The plugin did not flag the line as having a syntax warning or error, even if the Lua engine did not have an issue with the code per se.

### Reproduction steps

.

### Additional Notes

Apologies for raising another issue, but I couldn't find any information on if this is a niche feature of Lua or a bug that just hadn't been fixed in `5.1`. But if it's not a feature and is an unintended side-effort, I feel like it should be raised as a warning by the LLS to alert the user like "hey, are you sure this is what you were intending to do" similar to variables lacking a `nil`.

### Log File

_No response_

Reproduce the report in the Visual Studio Code extension using the Lua snippet `local varA, varB functionCalledHere()`. Start by tracing how syntax diagnostics are produced for this incomplete declaration, then add a regression test for the snippet and verify that a warning or error is reported rather than no diagnostic.

Written by the indexing model from the issue text.