query `@supports` not supported

[chharvey]

Write the following in a Less file:

// styles.less
@supports (color: green) {
  h1 { color: green; }
}

Run $ harp server and load a page with <link href="styles.css"/> in the head. The following error is received:

(node:63081) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): BrowserslistError: Unknown browser query `op_mini all`

See @supports for info.

This is not an issue with Less, as no error is received when compiling the Less file into CSS.