Added a format function to the standard library.

This is a wrapper around string.format so whatever format strings apply there apply here.
Due to a limitation of yarn functions you can only pass in a single argument parameter, for now.
Fixes #348

YarnSpinner.LanguageServer/src/Server/Documentation/BuiltInFunctionsAndCommands.ysls.json

@@ -282,6 +282,27 @@
         }
       ],
       "ReturnType": "number"
+    },
+    {
+      "YarnName": "format",
+      "DefinitionName": "format",
+      "Documentation": "Formats the argument parameter into the format_string. Intended for user facing text. Is a wrapper around String.Format and uses the format string rules from that.",
+      "Signature": "format(format_string, argument)",
+      "Parameters": [
+        {
+          "Name": "format_string",
+          "Type": "string",
+          "Documentation": "The format string that argument is to be injected into. Must follow C# string format rules.",
+          "IsParamsArray": false
+        },
+        {
+          "Name": "argument",
+          "Type": "any",
+          "Documentation": "The value to be injected into the format_string",
+          "IsParamsArray": false
+        }
+      ],
+      "ReturnType": "string"
     }
   ]
 }

YarnSpinner/Dialogue.cs

@@ -1209,6 +1209,11 @@ public StandardLibrary()
                     return Math.Truncate(num);
                 });
 
+                this.RegisterFunction("format", delegate (string formatString, object argument)
+                {
+                    return string.Format(System.Globalization.CultureInfo.CurrentCulture, formatString, argument);
+                });
+
                 #endregion Operators
             }
         }