add fallback behavior for showOpenFilePicker and showSaveFilePicker (#967)

## Launch Checklist

<!-- Thanks for the PR! Feel free to add or remove items from the
checklist. -->


 - [x] Briefly describe the changes in this PR.
 - [x] Link to related issues.
- [x] Include before/after visuals or gifs if this PR includes visual
changes.
 - [ ] Write tests for all new functionality.
 - [x] Add an entry to `CHANGELOG.md` under the `## main` section.

## Description

`showOpenFilePicker` and `showSaveFilePicker` are undefined on Firefox.
With this pr, Maputnik uses the old behavior as a fallback. It keeps the
naming "open" and "save" instead of "upload" and "download" to underline
that the style stays within the browser and no actual upload happens.

@zstadler Could you give it a try, please?

## Related Issue

- fixes https://github.com/maplibre/maputnik/issues/966

## Visual Changes

The "Save as" button gets hidden if `showSaveFilePicker` is not
available since it would have no use.

<table>
<tr>
<td>
Chrome
</td>
<td>
Firefox
</td>
</tr>
<tr>
<td>
<img
src="https://github.com/user-attachments/assets/cdc2cd4d-1c09-4dec-8c94-f8b0dd0c5b8e"
/>
</td>
<td>
<img
src="https://github.com/user-attachments/assets/0763ef63-6501-4cc1-977b-94753c3008ae"
/>
</td>
</tr>
</table>

CHANGELOG.md

@@ -6,7 +6,7 @@
 - Add scheme type options for vector/raster tile
 - Add `tileSize` field for raster and raster-dem tile sources
 - Update Protomaps Light gallery style to v4
-- Add support to edit local files on the file system
+- Add support to edit local files on the file system if supported by the browser
 - _...Add new stuff here..._
 
 ### 🐞 Bug fixes