Add warning for rules mixing positional and named references

[ydah]

Add a new MixedReferences warning that detects rules where positional references and named references are used together in the same rule's semantic actions.

No warning (positional references only)

expr: NUM NUM
{
$$ = $1 + $2;
}
;

Warnings

expr[result]: NUM[left] NUM[right]
{
$result = $1 + $right;
}
;

also warning:

expr[result]: NUM
{
$1;
}
NUM[right]
{
$result = $right;
}
;