va-button: adding independent chevron icon prop [WIP]

[Dr-Pongo]

Chromatic

https://va-button-icon-prop--60f9b557105290003b387cd5.chromatic.com

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Description

Test PR to see if this is a viable change that could benefit with button customization in the forms system by adding chevronLeft and chevronRight props to va-button which work with the existing back/continue prop functionality for displaying the left/right chevron icons in the button text. I'm not super hip with component library changes, so if there needs to be adjustments feel free to let me know!

Some background:
We use a number of custom pages and based on accessibility feedback it's beneficial to have custom text in the progression buttons that convey what page is coming up next. This has resulted in some extensive customization to get buttons like the following implemented and this change could help in a major way! This is mostly as a response to the feedback we received on some heavy styling changes we're looking to implement.

QA Checklist

• Component maintains 1:1 parity with design mocks
• Text is consistent with what's been provided in the mocks
• Component behaves as expected across breakpoints
• Accessibility expert has signed off on code changes (if applicable. If not applicable provide reason why)
• Designer has signed off on changes (if applicable. If not applicable provide reason why)
• Tab order and focus state work as expected
• Changes have been tested against screen readers (if applicable. If not applicable provide reason why)
• New components are covered by e2e tests; updates to existing components are covered by existing test suite
• Changes have been tested in vets-website using Verdaccio (if applicable. If not applicable provide reason why)

Screenshots

Acceptance criteria

• QA checklist has been completed
• Screenshots have been attached that cover desktop and mobile screens

Definition of done

• Documentation has been updated, if applicable
• A link has been provided to the originating GitHub issue (or connected to it via ZenHub)

[Dr-Pongo]

@micahchiang from our discussion earlier!

CC: @amponce @OptionSelect @josephrlee

[OptionSelect]

LGTM! 🚀