Fix workaround for SDFG file icons

[tbennun]

We need to keep track of microsoft/vscode#14662. Once it is resolved we can extend the existing icon set instead of copying the entire Seti webfont and .json

[tbennun]

Tracking microsoft/vscode#110830 now.

[tbennun]

Tracking microsoft/vscode#118846 now.

[tbennun]

This is now merged as a proposed API. Need to investigate potential usage.