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.

mentioned in issue docs#391 (closed)

Postgres IS documentation: https://www.postgresql.org/docs/current/functions-comparison.html

requested review from @darthunix

changed the description

1. Насчет того, что тарантул поддерживает IS [NOT] только для NULL/UNKNOWN, а для TRUE/FALSE нет: мне потравилось твое решение подменять TRUE/FALSE на булевый оператор.
2. Я предлагаю расширить тесты, чтобы были покрыты:
   • a is null/unknown/true/false
   • a is not null/unknown/true/false

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• f84401ec - feat: support IS [NOT] expression

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