Sync keywords / reserved words from spec with the ones used in the implementation

### What is wrong?

The spec defines a list of keywords and reserved words but we the implementation doesn't yet respect those and it may be out of date.

### How can it be fixed

1. Make sure the spec lists all the keywords that Fe is currently using
2. Decide on the final keywords/reserved words in the spec
3. Ensure the implementation enforces the keywords/reserved words from the spec

The issue names no files or tests. Start by locating the keyword and reserved-word lists in the Fe specification and implementation, then compare them to identify discrepancies. Define the final list with the project maintainers, update both sources, and verify that the implementation enforces the agreed terms.

Written by the indexing model from the issue text.