Document the "is" keyword/operator.

The "is" keyword (or operator) has been available since Haxe 4.0, but it isn't mentioned at all in the manual.

- [ ] Document "is" on the keyword page. It is not as restricted as the other keywords in that it is still usable as an identifier.
- [ ] Document "is" on the binary operators page, in the last section next to the arrow operator.

Locate the manual's keyword page and binary operators page, then review the surrounding sections and their formatting. Document the "is" keyword on both pages, including its availability since Haxe 4.0 and identifier usage, and verify that both checklist items are addressed.

Written by the indexing model from the issue text.