Add link popup "URL" to translatable strings

So people can add a different label if they want to.
I've made it backwards compatible by adding the `|| 'URL'` since no translation file has this string yet.

People can then override this string by overriding the translation. eg:
`
($ as any).trumbowyg.langs.nl = {
	...($ as any).trumbowyg.langs.nl,
	url: 'Link',
	target: 'Open in',
};
`

src/trumbowyg.js

@@ -1276,7 +1276,7 @@ Object.defineProperty(jQuery.trumbowyg, 'defaultOptions', {
 
  var options = {
  url: {
- label: 'URL',
+ label: t.lang.linkUrl || 'URL',
  required: true,
  value: url
  },