KQL: require operator and change operator grammar a bit

[zkat]

Ref: https://github.com/kdl-org/kdl/pull/204#issuecomment-935378397

This just makes it so "operators" are required, meaning we don't have to worry about syntax ambiguity with bare identifiers anymore :)

[zkat]

cc @tabatkins @larsgw I'd love your feedback

[larsgw]

Looks good to me, did you see this page from the CSS WG being tweeted yesterday? https://wiki.csswg.org/ideas/mistakes

Incomplete List of Mistakes in the Design of CSS

...

• descendant combinator should have been » and indirect sibling combinator should have been ++, so there's some logical relationships among the selectors' ascii art