Add option to disable "modified" field in notes

[tetafro]

Feature description

This is how currently a header of a note looks like:

---
title: Example
tags: [tag1]
created: 2019-09-21T09:40:23.688Z
modified: 2024-01-15T16:50:26.239Z
---

I'd like to disable modified field which is updated on each note save.

Feature motivation

I use git for syncing notes across devices, and this line is a frequent source of conflicts.