Merge #11598

11598: feat: Parse destructuring assignment r=Veykril a=ChayimFriedman2

Part of #11532.

Lowering is not as easy and may not even be feasible right now as it requires generating identifiers: `(a, b) = (b, a)` is desugared into
```rust
{
    let (<gensym_a>, <gensym_b>) = (b, a);
    a = <gensym_a>;
    b = <gensym_b>;
}
```

rustc uses hygiene to implement that, but we don't support hygiene yet.

However, I think parsing was the main problem as lowering will just affect type inference, and while `{unknown}` is not nice it's much better than a syntax error.

I'm still looking for the best way to do lowering, though.

Fixes #11454.

Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>

crates/syntax/src/tests/ast_src.rs

@@ -143,6 +143,7 @@ pub(crate) const KINDS_SRC: KindsSrc = KindsSrc {
         "RETURN_EXPR",
         "YIELD_EXPR",
         "LET_EXPR",
+        "UNDERSCORE_EXPR",
         "MATCH_EXPR",
         "MATCH_ARM_LIST",
         "MATCH_ARM",