Add Lua language server config and VS Code extension recommendation

.luarc.json declares the hl global so sumneko.lua doesn't flag
hyprland.lua's hl.* calls as undefined globals; extensions.json
recommends the sumneko.lua extension itself.

.luarc.json

@@ -0,0 +1,3 @@
+{
+    "diagnostics.globals": ["hl"]
+}

.vscode/extensions.json

@@ -0,0 +1,3 @@
+{
+    "recommendations": ["sumneko.lua"]
+}