[PDLL] Add support for `op` Operation expressions

An operation expression in PDLL represents an MLIR operation. In
the match section of a pattern, this expression models one of
the input operations to the pattern. In the rewrite section of
a pattern, this expression models one of the operations to
create. The general structure of the operation expression is very
similar to that of the "generic form" of textual MLIR assembly:

```
let root = op<my_dialect.foo>(operands: ValueRange) {attr = attr: Attr} -> (resultTypes: TypeRange);
```

For now we only model the components that are within PDL, as PDL
gains support for blocks and regions so will this expression.

Differential Revision: https://reviews.llvm.org/D115296