Syntax highlighting moved into a plugin directory

Syntax highlighting was moved out of the UI module and into a dedicated plugin directory, giving it a stable and extensible home.

Where it lives now

lib/harper-ui/src/plugins/
└── syntax/

Widgets import the highlighting helper through the plugin path, and the main binary consumes the shared library through harper:: references rather than re-declaring modules. The library exports the plugins module, so the UI no longer declares syntax directly.

The motivation is that a plugin directory can hold more than highlighting later, without the UI crate owning unrelated presentation concerns.

Note

The original version of this post referenced src/plugins/syntax at the
repository root. The workspace was later reorganized into lib/harper-*
crates, so the correct path is lib/harper-ui/src/plugins/syntax.

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