feat: use captureStr

[fgdorais]

I thought your parser capture idea was really cool, so I added it to lean4-parser. Since I grabbed the name, this breaks your code. Here are proposed fixes.

[fgdorais]

I'm thinking I was too quick picking this capture API. I think you're use case where the parser output is dropped is probably a primary use. I'm thinking about revising the API with two interfaces: withCapture and dropCapture. What do you think? That way, dropCaptureString would be a drop-in replacement for your original capture parser.