Parse destructuring assignment

The only patterns we should parse are `..` in structs and `_`: the rest are either not supported or already valid expressions.

crates/parser/src/syntax_kind/generated.rs

@@ -188,6 +188,7 @@ pub enum SyntaxKind {
     RETURN_EXPR,
     YIELD_EXPR,
     LET_EXPR,
+    UNDERSCORE_EXPR,
     MATCH_EXPR,
     MATCH_ARM_LIST,
     MATCH_ARM,