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-command/src/core_commands/if_.rs

@@ -28,7 +28,13 @@ impl Command for If {
             )
             .optional(
                 "else_expression",
-                SyntaxShape::Keyword(b"else".to_vec(), Box::new(SyntaxShape::Expression)),
+                SyntaxShape::Keyword(
+                    b"else".to_vec(),
+                    Box::new(SyntaxShape::OneOf(vec![
+                        SyntaxShape::Block,
+                        SyntaxShape::Expression,
+                    ])),
+                ),
                 "expression or block to run if check fails",
             )
             .category(Category::Core)