Sniff idea: Detect when constant is being used in constant()

## Describe the solution you'd like

Not sure if VIPCS is the best way forward, but I think this would be a worthy sniff.

When using `constant()`, we should throw a warning if a string is not being inputted. Of course it is valid PHP to do something like:

```
constant( FOO_BAR );
```

But I think most likely, what the user wants to do is:

```
constant( 'FOO_BAR' );
```

## What code should be reported as a violation?

```
constant( FOO_BAR );
```

## What code should *not* be reported as a violation?

```
constant( 'FOO_BAR' );
constant( "FOO_BAR" );
constant( $foo_bar );
```