use generic types for cy.then and cy.get aliases

[antonkri97]

Hey all!

This is a small pr that, it seems to me, slightly improves the typing in Cypress when using aliases.

This is a move away from type assertion to a slightly more formal system.

This can help developers immediately see the format for working with aliases in their projects.

I was inspired by this https://github.com/cypress-io/cypress/issues/8762#issuecomment-1192080000.

This is how I made it work in my application. https://github.com/antonkri97/family/pull/7/files#diff-ecc0256f03e3c2246926d3533817ed8df3416df522a4edee7b89a27a972d56a8

I moved all the aliases into a separate file. If you think it's a good idea, I could put these types in a separate place too.

Thanks :)