Add basic lua scripting capabilities (#4341)

The scripting capabilities is locked behind a cmake flag, and is not enabled by default.

Co-authored-by: nerix <nerixdev@outlook.de>
Co-authored-by: pajlada <rasmus.karlsson@pajlada.com>

docs/chatterino.d.ts

@@ -0,0 +1,22 @@
+/** @noSelfInFile */
+
+declare module c2 {
+  enum LogLevel {
+    Debug,
+    Info,
+    Warning,
+    Critical,
+  }
+  class CommandContext {
+    words: String[];
+    channel_name: String;
+  }
+
+  function log(level: LogLevel, ...data: any[]): void;
+  function register_command(
+    name: String,
+    handler: (ctx: CommandContext) => void
+  ): boolean;
+  function send_msg(channel: String, text: String): boolean;
+  function system_msg(channel: String, text: String): boolean;
+}