[feat] custom .desktop file template

[snorkysnark]

Describe the problem

I'm trying to implement a custom URL scheme in my app, and as demonstrated by #323 (comment) this is already possible on Windows using a custom wix template.
It would be pretty easy to do on Linux as well, but it requires some changes to the .desktop file: namely adding a MimeType field and appending %u after Exec:

[Desktop Entry]
Categories=Development;
Exec=app-name %u
Icon=app-name
Name=app-name
Terminal=false
Type=Application
MimeType=x-scheme-handler/customprotocol

Describe the solution you'd like

Use a handlebars template for .desktop file generation, the same way it works for wix:

"deb": {
    "depends": [],
    "desktopTemplate": ""
},

[Desktop Entry]
Categories={{{categories}}}
Exec={{{exec}}}
Icon={{{icon}}}
Name={{{name}}}
Terminal=false
Type=Application

I can make a PR later if you aggree on this change

Alternatives considered

No response

Additional context

No response

[amrbashir]

A PR would be greatly appreciated, thanks.