refactor: breadcrumbs base styles and themes

[jouni]

This is how I would want the component styles to be. Not sure what we need to change in the specs to get closer to this.

Changes on a high level

• Add padding around items, so that they gain a bigger click target, and that the focus outline is offset from the text.
• Use baseline alignment, so that in case of the text in an item doesn't fit the width and wraps, the any prefix icon and other items in the breadcrumbs are still aligned on the top/first line.
  • Use the same white-space character for the icon pseudo elements, as in other similar cases, so that the icon element also sits on the baseline.
• Use a mask-image for the overflow button ellipsis icon, instead of the font-dependent … character.
• Don't clip/hide overflow, to avoid padding + negative margin to keep focus outlines visible. Ensure items still collapse to the overflow menu by setting width, min-width and max-width on the host and the internal list.
• Always use the pointer cursor for links, as that’s the expectation for links on the web.
• Remove the link underline by default in base styles, as both Lumo and Aura want that removed.
• Treat disabled items visually the same as items without a path (text-only items), to keep them legible.
• Minor adjustments for iOS (prevent default tap highlight color).
• Add a collection of --vaadin-breadcrumbs-* custom properties for easier customization.
• Aura and Lumo opt to use the secondary text color for the links by default, and offer a variant (accent and primary, respectively) for coloring the links, which makes it possible to distinguish between link and non-link items without interacting with them.
• Add a built-in slash variant to base styles, which changes the default separator icon from a chevron to a slash.

[web-padawan]

Created #11876 to solve this, PTAL.

[web-padawan]

Pushed an update to the spec to list new custom CSS properties, also fixed JSDoc.
I'll continue reviewing the actual implementation later and update specs if needed.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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