This Joplin plugin exposes CodeMirror's relevant settings to Joplin users, which enable switching between tab indentation and space indentation. In addition, it can automatically reformat your notes (if enabled in the settings) and convert any existing tabs to spaces (or vice versa, if you decide you're a tab-person after all). A dedicated toolbar button calls the same action manually.
Thanks to Rich Markdown (@CalebJohn), that heavily influenced this implementation.