Replace Popper.js with CSS Anchor Positioning API

Replace Popper.js with CSS Anchor Positioning API

BREAKING CHANGE: Popper.js is no longer a dependency. Dropdowns, tooltips,
and popovers now use native CSS Anchor Positioning with a polyfill fallback.

JavaScript:
- Add positioning.js utility with responsive placement support
- Rewrite dropdown.js to use anchor positioning and popover attribute
- Rewrite tooltip.js to use anchor positioning
- Update popover.js with data-api click handler
- Remove Popper.js imports and configuration options
- Add responsive placement syntax (e.g., "bottom md:top lg:right-start")

SCSS:
- Add _anchor-positioning.scss with base styles for positioned elements
- Update dropdown, tooltip, and popover styles for anchor positioning
- Add b-dropdown custom element support

Package:
- Replace @popperjs/core with @oddbird/css-anchor-positioning (optional polyfill)

Docs:
- Update dropdowns, tooltips, and popovers documentation