Parser refactoring for improving pipelines

[sophiajt]

Description

This is a batch of parser refactors for moving away from pipelines that assume things are always piped to being able to handle more bash-isms in the pipelines.

This PR does not add additional functionality, but instead makes adding that functionality in the future easier. I've put in a few potential cases for pipeline elements we might want, though this list may change as we decide which bashisms we want.

User-Facing Changes

None yet.

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 --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra 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.