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>

THIRD-PARTY.md

@@ -42,10 +42,18 @@ SOFTWARE.
 Colour values follow the Gruvbox palette by Pavel Pertsev:
 https://github.com/morhetz/gruvbox — MIT/X11 License.
 
-## Not included: syntax highlighting theme
+## Syntax highlighting theme
 
-This repository deliberately ships **no** `.tmTheme` file. The Gruvbox
-`.tmTheme` commonly paired with Yazi comes from
-https://github.com/poperigby/gruvbox-dark-yazi, which is licensed **AGPL-3.0**.
-Bundling it would force AGPL-3.0 on this repository. Fetch it yourself if you
-want Gruvbox-coloured file previews — see the README.
+`gruvbox-dark.tmTheme` is original work, written for this repository and
+covered by its MIT licence. It is not derived from any existing `.tmTheme`.
+
+Its scope-to-colour mapping follows the highlight links defined in
+morhetz/gruvbox's own vim colorscheme (Comment=gray, String=green,
+Constant=purple, Statement/Keyword=red, Identifier=blue, Function=green bold,
+Type=yellow, StorageClass=orange, PreProc=aqua, Operator=Normal) — a mapping
+rule, not copied code.
+
+The Gruvbox `.tmTheme` commonly paired with Yazi comes from
+https://github.com/poperigby/gruvbox-dark-yazi and is licensed **AGPL-3.0**.
+It is deliberately **not** used or bundled here: doing so would force
+AGPL-3.0 on this repository.