[core][FR] Kbd component

[ggdouglas]

Description

Blueprint has KeyComboTag for rendering hotkey combos, but no standalone component for displaying a styled <kbd> element. KeyComboTag is tightly coupled to the hotkey system; it parses combo strings through normalizeKeyCombo, maps platform-specific icons, etc. If you just want to show a keyboard key in a tooltip or some inline text, there's no good way to do that today.

We already have the CSS for this (.bp6-key, .bp6-key-combo) and KeyComboTag renders <kbd> elements internally, but that styling isn't usable on its own.

Prior art

• shadcn/ui Kbd - Kbd for single keys, KbdGroup for combinations
• Mantine Kbd - simple Kbd with size prop (xs through xl)
• Chakra UI Kbd - semantic <kbd> wrapper with theme integration

All three are thin wrappers around the HTML <kbd> element with consistent styling.

Possible API

// Single key
<Kbd>⌘</Kbd>

// Combination (could reuse existing .bp6-key-combo styles)
<KbdCombo>
  <Kbd>Ctrl</Kbd>
  <Kbd>Shift</Kbd>
  <Kbd>K</Kbd>
</KbdCombo>

// Or as a simple string-based API similar to KeyComboTag
<KbdCombo keys={["Ctrl", "Shift", "K"]} />

Context

Came up during review of #7237. KeyComboTag internally renders <kbd> elements but isn't designed for general use outside the hotkeys system.