Add darkModeSupport to allow dark themed title bar.

[vplme]

Adding "darkModeSupport": true to the configuration adds an item to the Info.plist in the Mac app which allows the titlebar of Riot to change with the System theme (light, dark).

In electron-builder:

    if (this.platformSpecificBuildOptions.darkModeSupport) {
      appPlist.NSRequiresAquaSystemAppearance = false
    }

Source: https://github.com/electron-userland/electron-builder/blob/faba9e365a0a4d2402d91ca9bd5a5288cb78f0f7/packages/app-builder-lib/src/macPackager.ts#L313-315

Electron info: https://electronjs.org/docs/tutorial/mojave-dark-mode-guide

Before:

After:

[t3chguy]

Please read the contributing guide: https://github.com/vector-im/riot-web/blob/develop/CONTRIBUTING.rst

Needs sign off

[vplme]

Signed-off-by: Victor Verstappen <vpl@vpl.me>

Did not realise that it also applied to configuration files since it doesn't seem like a licensable contribution.

[t3chguy]

Looks awesome, thanks for your contribution

Related to #8394