Fix `takeDelimiter` and `takeDelimiterExclusive` tossing too many bytes
(#25132)

Also add/improve test coverage for all delimiter and sentinel methods,
update usages of `takeDelimiterExclusive` to not rely on the fixed bug,
tweak a handful of doc comments, and slightly simplify some logic.

I have not fixed #24950 in this commit because I am a little less
certain about the appropriate solution there.

Resolves: #25132

Co-authored-by: Andrew Kelley <andrew@ziglang.org>