Add OneOf shape to fix else (#7385)

# Description

fixes #7384 

This is a stop-gap fix until we remove type-directed parsing. With this,
you can create a `OneOf` shape that can be one of a list of syntax
shapes. This gives you a little more control than you get with `Any`,
allowing you to add `Block` without breaking other parsing rules.

# User-Facing Changes

`else` block will no longer capture variables as it will now use a block
instead of a closure.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-parser/src/parser.rs

@@ -644,6 +644,27 @@ pub fn parse_multispan_value(
 
             (arg, error)
         }
+        SyntaxShape::OneOf(shapes) => {
+            for shape in shapes.iter() {
+                if let (s, None) = parse_multispan_value(
+                    working_set,
+                    spans,
+                    spans_idx,
+                    shape,
+                    expand_aliases_denylist,
+                ) {
+                    return (s, None);
+                }
+            }
+            let span = spans[*spans_idx];
+            (
+                Expression::garbage(span),
+                Some(ParseError::Expected(
+                    format!("one of a list of accepted shapes: {:?}", shapes),
+                    span,
+                )),
+            )
+        }
         SyntaxShape::Expression => {
             trace!("parsing: expression");
 
@@ -4252,7 +4273,10 @@ pub fn parse_value(
             } else {
                 return (
                     Expression::garbage(span),
-                    Some(ParseError::Expected("non-block value".into(), span)),
+                    Some(ParseError::Expected(
+                        format!("non-block value: {}", shape),
+                        span,
+                    )),
                 );
             }
         }