Allow using "with" keyword in "let" statements

Askama uses the syntax `{% when Variant with (parameters) %}` in
`{% match %}` blocks.

This change allows the optional use of the keyword "with" in "let" and
"if let" statements, too.

askama_shared/src/parser.rs

@@ -442,6 +442,7 @@ fn target(i: &[u8]) -> IResult<&[u8], Target<'_>> {
     // match structs
     let (i, path) = opt(path)(i)?;
     if let Some(path) = path {
+        let (i, _) = opt(ws(tag("with")))(i)?;
         let (i, is_unnamed_struct) = opt_opening_paren(i)?;
         if is_unnamed_struct {
             let (i, targets) = alt((

testing/tests/let_destructoring.rs

@@ -105,3 +105,17 @@ fn test_let_destruct_with_path() {
     };
     assert_eq!(t.render().unwrap(), "hello");
 }
+
+#[derive(Template)]
+#[template(source = "{% let some::path::Struct with (v) = v %}{{v}}", ext = "txt")]
+struct LetDestructoringWithPathAndWithKeyword<'a> {
+    v: some::path::Struct<'a>,
+}
+
+#[test]
+fn test_let_destruct_with_path_and_with_keyword() {
+    let t = LetDestructoringWithPathAndWithKeyword {
+        v: some::path::Struct("hello"),
+    };
+    assert_eq!(t.render().unwrap(), "hello");
+}