internal: move ws attachment logic to the parser crate

This has to re-introduce the `sink` pattern, because doing this purely
with iterators is awkward :( Maaaybe the event vector was a false start?

But, anyway, I like the current factoring more -- it sort-of obvious
that we do want to keep ws-attachment business in the parser, and that
we also don't want that to depend on the particular tree structure. I
think `shortcuts` module achieves that.

crates/parser/src/lib.rs

@@ -26,6 +26,7 @@ mod parser;
 mod grammar;
 mod input;
 mod output;
+mod shortcuts;
 
 #[cfg(test)]
 mod tests;
@@ -36,6 +37,7 @@ pub use crate::{
     input::Input,
     lexed_str::LexedStr,
     output::{Output, Step},
+    shortcuts::StrStep,
     syntax_kind::SyntaxKind,
 };