bat + yazi flavors: sync configs, external the purescript syntax clone

- dot_config/bat: config + cyberdream theme synced; embedded
  sublime-purescript-syntax git clone (with dot_git) replaced by a
  git-repo external (clone-when-missing, ff-only)
- run_onchange bat cache --build hook keyed on theme/config
- yazi cyberdream flavor synced; flavors un-ignored in .chezmoiignore

.chezmoiexternal.toml

@@ -8,3 +8,10 @@
     url = "https://github.com/prdlk/nvim.git"
     [".config/nvim".pull]
         args = ["--ff-only"]
+
+# bat syntax definition — upstream git repo, cloned when missing.
+[".config/bat/syntaxes/sublime-purescript-syntax"]
+    type = "git-repo"
+    url = "https://github.com/tellnobody1/sublime-purescript-syntax.git"
+    [".config/bat/syntaxes/sublime-purescript-syntax".pull]
+        args = ["--ff-only"]