Issue when using tab key on readOnly code editor

[cyrilf]

When using the tab key on a readOnly editor, this error occurs:

Uncaught TypeError: this.content is undefined
    tab CodeEditor.vue:283

To solve it, we need to change these lines:
https://github.com/justcaliturner/simple-code-editor/blob/master/npm-package/CodeEditor.vue#L282-L283

+ if (this.content) {
    this.content =
       this.content.substring(0, cursorPosition) + this.tabWidth + this.content.substring(cursorPosition); 
+ }

or another option, do a:
if (this.readOnly) return
as the first line of the tab method.