context-aware tokenization: Sometimes >> is better parsed as two copies of > (similar for ]] ) #2092

Example for the >> case:

[[block]] struct B { a:  array<array<i32,2>>; } 
// That >> should two copies of GREATER_THAN, not one copy of SHIFT_RIGHT

Example for the ]] case:

fn foo() {
  var a : array<i32,10>;
  let v = a[a[0]]; // That ]] should be two copies of BRACKET_RIGHT instead of ATTR_RIGHT.
}

Tint parses these provisionally as the longer token, then backtracks to split them if needed.

I think the group has already discussed this but I can't find the tracking issue.

Related: #1063 validating the grammar is unambiguous