Normative: add `expr is pattern` expression

[Jack-Works]

is expression

A new expression is expression cherry picked from the recent discussion about pattern matching.

Syntax

RelationalExpression `is` MatchPattern

This syntax has the same priority as instanceof and in expression.

Semantics

Check if the LHS expression match the RHS pattern, returns a boolean.

It does not introduce new bindings in the current scope.

Note

The MatchPattern and its semantics is unmodified, therefore it have some unintuitive behaviors like:

if (x is y) {
    // always runs, because y is a always success match.
}

if (z is { type }) {
    type // ReferenceError. is expression does not introduce new bindings.
}

Syntax bikeshedding

Maybe expr matches pattern?