Implement `map.exists(key)` operation

[Gusarich]

All of the code was generated in Copilot Workspace: https://github.com/tact-lang/tact/issues/578#issuecomment-2229226709

Closes #578

• [x] I have updated CHANGELOG.md
• [ ] I have documented my contribution in Tact Docs: https://github.com/tact-lang/tact-docs/pull/PR-NUMBER
• [x] I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• [x] I have run all the tests locally and no test failure was reported
• [x] I have run the linter, formatter and spellchecker
• [x] I did not do unrelated and/or undiscussed refactorings

[anton-trunov]

Let's resolve the merge conflicts and see how well Copilot is doing here :)