Fix: Add viewer hooks and autocmds to hover popup for consistent highlighting

Problem:

Currently, the hover popup only calls `helper.fern.renderer.highlight()`
and `helper.fern.renderer.syntax()`, which means user customizations
made through `FernHighlight`/`FernSyntax` autocmds or
`viewer:highlight`/`viewer:syntax` hooks are not applied to the hover
popups.

As a result, fern-specific customizations made in these hooks/autocmds
only affect the main window, creating visual inconsistency. This
behavior makes it difficult to customize highlighting that is not
defined by Fern, such as CursorLine, which is limited to fern.

See also: #283

Solution:

This PR adds `viewer:highlight`, `viewer:syntax` hooks and
`FernHighlight`, `FernSyntax` autocmds to the hover popup's `s:apply()`
function to ensure consistent highlighting between the main fern window
and hover popups.

Author: sirasagi62

autoload/fern/internal/drawer/hover_popup.vim

@@ -73,7 +73,11 @@ function! s:show() abort
   function! s:apply() abort closure
     call setbufline('%', 1, line)
     call helper.fern.renderer.syntax()
+    call fern#hook#emit('viewer:highlight', helper)
+    doautocmd <nomodeline> User FernHighlight
     call helper.fern.renderer.highlight()
+    call fern#hook#emit('viewer:syntax', helper)
+    doautocmd <nomodeline> User FernSyntax
     syntax clear FernRootSymbol
     syntax clear FernRootText