v2: Plugin architecture for custom components

[joeworkman]

Carry-over from closed PR #30 by @somewebmedia and issue #48.

v2 should support user-defined custom components without forking the core. This was the most-requested feature (13 comments on #48).

Related closed PRs/issues:

• PR Added possibility of extending and creating new elements #30 — Added possibility of extending and creating new elements
• PR Attributes for inner link in button and menu-item #94 — Attribute passthrough for button/menu-item inner elements
• Issue Is it possible to add custom tag / attribute ? #48 — Custom tag/attribute support
• Issue Button custom attributes not retained #111 — Button custom attributes not retained
• Issue Allow XML namespace prefixes for inky tags #91 — XML namespace prefixes for CMS integration

[joeworkman]

Implemented in v2 via the <ink-*> custom components system. Users create reusable components as .inky files in a components/ directory (configurable via inky.config.json). Attributes become template variables, inner content replaces <yield>, and components can nest. No need to fork core.

See docs: https://github.com/foundation/inky/blob/feature/2.0-rust/docs/getting-started.md#custom-components