refactor(Util.escapeMarkdown): allow separate escaping and add tests (#3241)

* wip refactor

* add escapeMarkdown tests

* italics can be done with a single underscore too

* more refined

* fix test name

* unnecessary eslint ignores

* use jest

* make eslint less annoying in this test file

* more testing

* fix lib usage

* more tests and a small fix

src/structures/APIMessage.js

@@ -93,7 +93,7 @@ class APIMessage {
     if (content || mentionPart) {
       if (isCode) {
         const codeName = typeof this.options.code === 'string' ? this.options.code : '';
-        content = `${mentionPart}\`\`\`${codeName}\n${Util.escapeMarkdown(content || '', true)}\n\`\`\``;
+        content = `${mentionPart}\`\`\`${codeName}\n${Util.cleanCodeBlockContent(content || '')}\n\`\`\``;
         if (isSplit) {
           splitOptions.prepend = `${splitOptions.prepend || ''}\`\`\`${codeName}\n`;
           splitOptions.append = `\n\`\`\`${splitOptions.append || ''}`;