refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

util.js

@@ -410,7 +410,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
         if (array) {
           str = str.split('\n').map(function(line) {
             return '  ' + line;
-          }).join('\n').substr(2);
+          }).join('\n').slice(2);
         } else {
           str = '\n' + str.split('\n').map(function(line) {
             return '   ' + line;
@@ -427,7 +427,7 @@ function formatProperty(ctx, value, recurseTimes, visibleKeys, key, array) {
     }
     name = JSON.stringify('' + key);
     if (name.match(/^"([a-zA-Z_][a-zA-Z_0-9]*)"$/)) {
-      name = name.substr(1, name.length - 2);
+      name = name.slice(1, -1);
       name = ctx.stylize(name, 'name');
     } else {
       name = name.replace(/'/g, "\\'")