Comment line feature with one key symbol (like the LaTeX "%")

### What
Make a symbol like "%" start a comment until the end of the current line. This could be optional of course.

### Why
It would be supernice to take advantage of this to quickly keep track of ideas while maintaining a neat output. It's one of the key advantages of the long-living LaTeX system. It would also increase compatibility with LaTeX users that are many in the academic field.

### How
Maybe implement automatic replacement of % with beginning of comment "" at the end of the current line before the markdown starts it's job of viewing the output.

The issue names no source file, test, or entry point. Trace the macOS editor's line-processing path and Markdown rendering entry point; verify how a single % should delimit comments and confirm the rendered output omits the comment while ordinary % usage remains unchanged.

Written by the indexing model from the issue text.