Throwaway idea: Content aware Save As

### Problem

Code already detect syntax, but we could extend capabilities depending on which content for, for example, a Saving As that already prefill the name depending on the content

### Proposal

Upon doing a save As of an unnamed file, tweak initial_name according to the content:

- if it has a list of URI, Path, URL, call the initial_name "New playlist.m3u8"
- if it has all lines with commas, or a specified delimiter specified as per csv standard, append a ".csv" in the "unsaved document" initial name
- if it has a shebang on the first line, add a .sh, or .py, or according to what it is
- if it has some html, add a .html extension
- Vala, it's a NewDocument.vala...

### Prior Art (Optional)

_No response_