Support destructuring assignments (RFC 2909)

The supported patterns are already valid as expressions, except the rest pattern (`..`) and the wildcard pattern (`_`).

Author: ChayimFriedman2

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "ungrammar"
 description = "A DSL for describing concrete syntax trees"
-version = "1.15.0"
+version = "1.16.0"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/matklad/ungrammar"
 edition = "2018"

rust.ungram

@@ -358,6 +358,7 @@ Expr =
 | WhileExpr
 | YieldExpr
 | LetExpr
+| UnderscoreExpr
 
 Literal =
   Attr* value:(
@@ -426,7 +427,7 @@ RecordExprFieldList =
   '{'
     Attr*
     fields:(RecordExprField (',' RecordExprField)* ','?)?
-    ('..' spread:Expr)?
+    ('..' spread:Expr?)?
   '}'
 
 RecordExprField =
@@ -500,6 +501,9 @@ YieldExpr =
 LetExpr =
   Attr* 'let' Pat '=' Expr
 
+UnderscoreExpr =
+  Attr* '_'
+
 AwaitExpr =
   Attr* Expr '.' 'await'