feat: add own Gruvbox tmTheme, drop the AGPL dependency

Syntax highlighting for the file preview no longer points at an external
repository. gruvbox-dark.tmTheme is written from scratch here: 39 scope rules
following morhetz/gruvbox's own vim highlight links (Comment=gray italic,
String=green, Constant=purple, Keyword=red, Identifier=blue, Function=green
bold, Type=yellow, StorageClass=orange, PreProc=aqua, Operator=Normal).

The theme commonly paired with Yazi is AGPL-3.0 and would have forced that
licence on this repository. Writing our own keeps everything MIT and makes the
repo self-contained.

Verified under syntect (the library Yazi uses): rendering a sample through bat
emits only palette colours, and a Yazi frame shows 14 distinct colours with no
foreign value.

regen-tmtheme.py reproduces the file byte-for-byte.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

theme.toml

@@ -36,10 +36,8 @@ count_selected = { fg = "#282828", bg = "#fabd2f" }
 # Border
 border_style = { fg = "#665c54" }
 
-# Syntax highlighting in the file preview.
-# Not shipped with this repo (the Gruvbox .tmTheme is AGPL-3.0, see THIRD-PARTY.md).
-# Fetch it as described in the README, then uncomment:
-# syntect_theme = "~/.config/yazi/Gruvbox-Dark.tmTheme"
+# Syntax highlighting in the file preview (gruvbox-dark.tmTheme ships with this repo)
+syntect_theme = "~/.config/yazi/gruvbox-dark.tmTheme"
 
 # : }}}