Add OneOf shape to fix else

[sophiajt]

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 after the PR is merged, if necessary. This will help us keep the docs up to date.