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>

src/controllers/plugins/LuaAPI.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#ifdef CHATTERINO_HAVE_PLUGINS
+
+struct lua_State;
+namespace chatterino::lua::api {
+// names in this namespace reflect what's visible inside Lua and follow the lua naming scheme
+
+// NOLINTBEGIN(readability-identifier-naming)
+// Following functions are exposed in c2 table.
+int c2_register_command(lua_State *L);
+int c2_send_msg(lua_State *L);
+int c2_system_msg(lua_State *L);
+int c2_log(lua_State *L);
+
+// These ones are global
+int g_load(lua_State *L);
+int g_print(lua_State *L);
+int g_import(lua_State *L);
+// NOLINTEND(readability-identifier-naming)
+
+// Exposed as c2.LogLevel
+// Represents "calls" to qCDebug, qCInfo ...
+enum class LogLevel { Debug, Info, Warning, Critical };
+
+}  // namespace chatterino::lua::api
+
+#endif