Suggestion: TypeScript: remove fallback for `should` to increase type safety

[OliverJAsh]

Current behavior:

cy.get('foo').should('whatevergoeshere'); // no error :-(

cy.get('foo').should('exis'); // oops, typo—but no error :-(
cy.get('foo').should('exist'); // ✅

Desired behavior:

cy.get('foo').should('whatevergoeshere'); // type error, `whatevergoeshere` is not a valid assertion

cy.get('foo').should('exis'); // type error, `exis` is not a valid assertion
cy.get('foo').should('exist'); // ✅