nvim: extract PlantUML JAR from launch script

I had previously hard-coded the path on my Debian install, but this
commit will find the correct JAR file as long as there is a `plantuml`
launch script in the PATH.

config/nvim/lua/fschauen/plugins/plantuml-previewer.lua

@@ -15,10 +15,12 @@ return {
   ft = "plantuml",
 
   init = function()
-    -- Override the bundled JAR if one in installed in `/usr/local`.
-    local path = "/usr/local/share/plantuml/plantuml.jar"
-    if vim.loop.fs_stat(path) then
-      vim.g["plantuml_previewer#plantuml_jar_path"] = path
+    -- Prefer the system PlantUML (if any) over the one bundled with the plugin.
+    local cmdline = [[which plantuml | xargs cat | grep plantuml.jar]]
+    local regex = [[\v\s['"]?(\S+/plantuml\.jar)]]
+    local jar = vim.fn.matchlist(vim.fn.system(cmdline), regex)[2]
+    if jar and vim.loop.fs_stat(jar) then
+      vim.g["plantuml_previewer#plantuml_jar_path"] = jar
     end
   end,
 }