[PLAY-1307] add Loading Inline custom text option

[ElisaShapiro]

What does this PR do? A clear and concise description with your runway ticket url.
PLAY-1307 adds a text prop with a default value of 'Loading' to the Loading Inline kit, and adds a Custom Text kit example showing developers how to customize their Loading Inline components with the new prop. Both the Rails and React versions of the kit have been updated, as have their tests.

In the course of this ticket I also standardized the primary Loading Inline example's name as "default" instead of "light" which was set ~5 years ago - the name is now "default" consistently across Rails and React files and kit example pages.

Screenshots: Screenshots of Custom Text kit example on Rails and React kit pages

How to test? Steps to confirm the desired behavior:

1. Go to Loading Inline kit page (rails and react).
2. Scroll down to 'Custom Text' kit example.
3. See added kit example and select '</> Show Code' to see how custom text values are set.

Checklist:

• LABELS Add a label: enhancement, bug, improvement, new kit, deprecated, or breaking. See Changelog & Labels for details.
• DEPLOY I have added the milano label to show I'm ready for a review.
• TESTS I have added test coverage to my code.