feature: allow custom placeholder values for expiry month and year

[SeanCassiere]

Primarily closes #6

Also:

• add: run-time validation for checking the undefined state of required props which was lost with the removal of the prop-types package.
• refactor: the formulation of the displayed card-number which was not easy to read.

[felquis]

I feel like you might want to check out this comment https://github.com/felquis/react-credit-cards-2/issues/7#issuecomment-1496775340 @SeanCassiere

[SeanCassiere]

I feel like you might want to check out this comment #7 (comment) @SeanCassiere

I looked into it, and I couldn't recreate the bug locally, where it was returning the issuer as unknown using the Visa test card number example.

For recreating, I tried typing in the numbers individually and also tried directly copy-pasting the test Visa card number (4242 4242 4242 4242). Please see below.

• Typing in the numbers individually for the card number 4242424242424242.

  

• Direct copy-pasting the number (4242 4242 4242 4242 and without spaces 4242424242424242)

  

  NOTE: The unknowns being shown in the console were onRender and onFocus, once the typing began, it showed the issuer as visa.

Testing was done on the current release version 1.0.1.

It's entirely possible that I'm missing something here, but I didn't touch the logic used for the callback function, just changed the file-extension to .ts.

[felquis]

I'm going to review this PR this week, anyone else is welcomed to review too.