feat: support IS [NOT] expression

Summarize the changes

• feat: support IS [NOT] expression

Closes #732 (closed)

Docs follow-up: docs#391 (closed)

Ensure that

• New code is covered by unit and integration tests.
• Related issues would be automatically closed with gitlab's closing pattern (Closes #issue_number).
• Public modules are documented (check the rendered version with cargo doc --open).
• (if PEST grammar has changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none
• (if EBNF grammar has changed) create a follow-up issue in picodata/docs.