Consider adopting codicons

[KamasamaK]

Some of the custom icons used here can be replaced with codicons, which is what VS Code uses natively.

In the manifest, an example of how this can be done is to replace https://github.com/hbenl/vscode-test-explorer/blob/8f9e2021dc29c2f1052afc2ab4796794322fa3c0/package.json#L205-L208 with

"icon": "$(refresh)"

An animated svg like for reloading can be replaced with $(refresh~spin)

Anywhere in the API that uses a ThemeIcon can be done with new ThemeIcon(codiconId)