Add map support for inline base64 fluent messages.

This enables the RC to parse and share custom messages
as part of the map's custom rules without any additional
API changes.

OpenRA.Mods.Common/Lint/CheckFluentReferences.cs

@@ -71,6 +71,11 @@ namespace OpenRA.Mods.Common.Lint
 					}
 				}
 			}
+
+			if (map.FluentMessageDefinitions.Nodes.Length > 0)
+				emitWarning(
+					$"Lint pass ({nameof(CheckFluentReferences)}) lacks the know-how to test inline map fluent messages " +
+					"- previous warnings may be incorrect");
 		}
 
 		void ILintPass.Run(Action<string> emitError, Action<string> emitWarning, ModData modData)