Re-add SilverStripe icon file (for custom use)

[purplespider]

Earlier this year, a SilverStripe icon was added for .ss files (#374), but was then removed in favour of a Scheme icon instead (#364).

Could the SilverStripe icon file please be re-included so that SilverStripe developers can choose to manually override it in settings with "*.ss": "silverstripe" if they wish to?

[PKief]

@purplespider sure, I'll add it back :)

[PKief]

With the next update it can be added back by this user setting:

"material-icon-theme.files.associations": {
    "*.ss": "silverstripe"
}

Result: