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-protocol/src/syntax_shape.rs

@@ -101,6 +101,9 @@ pub enum SyntaxShape {
     /// A custom shape with custom completion logic
     Custom(Box<SyntaxShape>, DeclId),
 
+    /// One of a list of possible items, checked in order
+    OneOf(Vec<SyntaxShape>),
+
     /// Nothing
     Nothing,
 }
@@ -132,6 +135,7 @@ impl SyntaxShape {
             SyntaxShape::Keyword(_, expr) => expr.to_type(),
             SyntaxShape::MathExpression => Type::Any,
             SyntaxShape::Number => Type::Number,
+            SyntaxShape::OneOf(_) => Type::Any,
             SyntaxShape::Operator => Type::Any,
             SyntaxShape::Range => Type::Any,
             SyntaxShape::Record => Type::Record(vec![]), // FIXME: What role should fields play in the Record type?
@@ -191,6 +195,11 @@ impl Display for SyntaxShape {
             SyntaxShape::Boolean => write!(f, "bool"),
             SyntaxShape::Error => write!(f, "error"),
             SyntaxShape::Custom(x, _) => write!(f, "custom<{}>", x),
+            SyntaxShape::OneOf(list) => {
+                let arg_vec: Vec<_> = list.iter().map(|x| x.to_string()).collect();
+                let arg_string = arg_vec.join(", ");
+                write!(f, "one_of({})", arg_string)
+            }
             SyntaxShape::Nothing => write!(f, "nothing"),
         }
     }