Add base64Encode handlebars helper

This is intended to pair with the browser atob function
so no base64Decode is provided,
but should be equivalent to the mtscript base64Encode
if data has already been encoded into the token's properties.

src/main/java/net/rptools/maptool/util/HandlebarsUtil.java

@@ -16,6 +16,8 @@
 
 import com.github.jknack.handlebars.Context;
 import com.github.jknack.handlebars.Handlebars;
+import com.github.jknack.handlebars.Helper;
+import com.github.jknack.handlebars.Options;
 import com.github.jknack.handlebars.Template;
 import com.github.jknack.handlebars.context.JavaBeanValueResolver;
 import com.github.jknack.handlebars.helper.AssignHelper;
@@ -29,8 +31,10 @@
 import java.io.File;
 import java.io.IOException;
 import java.net.URL;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Path;
 import java.util.Arrays;
+import java.util.Base64;
 import net.rptools.maptool.model.Token;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -95,6 +99,29 @@ protected URL getResource(String location) throws IOException {
     }
   }
 
+  private static enum MapToolHelpers implements Helper<Object> {
+    /**
+     * Turns the textual form of the value into a base64-encoded string. For example:
+     *
+     * <pre>
+     * &lt;script type="application/json;base64" id="jsonProperty"&gt;
+     *   {{ base64Encode properties[0].value }}
+     * &lt;/script&gt;
+     * &lt;script type="application/javascript"&gt;
+     * const jsonProperty = JSON.parse(atob(document.getElementById("jsonProperty").innerText));
+     * &lt;/script&gt;
+     * </pre>
+     */
+    base64Encode {
+      @Override
+      public Object apply(final Object context, final Options options) {
+        byte[] message = context.toString().getBytes(StandardCharsets.UTF_8);
+
+        return new Handlebars.SafeString(Base64.getUrlEncoder().encodeToString(message));
+      }
+    }
+  }
+
   /**
    * Creates a new instance of the utility class.
    *
@@ -111,6 +138,7 @@ private HandlebarsUtil(String stringTemplate, TemplateLoader loader) throws IOEx
       NumberHelper.register(handlebars);
       handlebars.registerHelper(AssignHelper.NAME, AssignHelper.INSTANCE);
       handlebars.registerHelper(IncludeHelper.NAME, IncludeHelper.INSTANCE);
+      Arrays.stream(MapToolHelpers.values()).forEach(h -> handlebars.registerHelper(h.name(), h));
 
       template = handlebars.compileInline(stringTemplate);
     } catch (IOException e) {