Support custom save path and name for exported files

[daishu0000]

Currently, when exporting a GeoJSON file from geojson.io by clicking the download button, the file is saved automatically with a default filename "map.geojson" to the browser's default download folder.

Issue:
I often want to specify a custom file name and a custom save location for the exported GeoJSON file.
The lack of this flexibility requires me to rename or move files manually after each export, which is inconvenient and inefficient, especially when working with multiple maps or projects.

Proposed Improvement:
Allow the user to choose the save location and name via the browser's native "Save As" dialog.

[tnkuehne]

Allow the user to choose the save location and name via the browser's native "Save As" dialog.

Programmatically triggering this is only available in Chromium based browsers (see https://web.dev/patterns/files/save-a-file), so limited support but could be done in an progressive enhancement way as outlined in the linked article. You could implement this by editing src/ui/file_bar.js.

Currently, when exporting a GeoJSON file from geojson.io by clicking the download button, the file is saved automatically with a default filename "map.geojson" to the browser's default download folder.

What works in Firefox and Chrome is to change the settings for file downloads and activate "Always ask you where to save files"/"Ask where to save each file before downloading".