Gradients and shadows

As a special add-on to Beta 2, I've taken a swing at properly addressing gradients and shadows for folks. This PR is a huge update to how we utilize shadows and gradients across Bootstrap (when they're enabled, at least). Let's start with what issues this addresses:

• Fixes #24389 (closed): Always forces a focus state box-shadow on inputs and buttons, regardless of $enable-shadows, to ensure proper accessibility.
• Fixes #24242 (closed): Adds variables for the shadow width and color.
• Closes #23837 (closed): Improves shadow styles for inputs and buttons, though this isn't always 100% given some shadows are designed to work with gradients.
• Fixes #21367 (closed): Finally adds gradients to buttons, woohoo!

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Now, here's what's changed to address those issues.

For shadows

• Replaced $btn-focus-box-shadow and $input-focus-box-shadow with a new unified variable to match our $input-btn- approach: $input-btn-focus-box-shadow.
• Added two new variables to control the size of the focus shadow and the color: $input-btn-focus-width and $input-btn-focus-color.
• Changed the focus shadow's width from 3px to .2rem.
• For accessibility and consistency with validation styles, updated the form-control-focus() mixin to use a manual $enable-shadows check so we can still provide a focus state style despite the global shadow override.
• Updated the button-variant() and button-outline-variant() mixins to use the new $input-btn-focus-width variable, replacing static 3px values throughout.

For gradients

• Added new gradient-bg() mixin for specifying an opt-in gradient with a background-color fallback.
• Added opt-in gradients to alerts, buttons, carousel controls, custom checks and file inputs, and dropdown items.
• Added new .bg-gradient- utilities when $enable-gradients is set to true (set to false by default).

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