New command: `ovsx verify-pat`

[felipecrs]

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The vsce verify-pat exists since https://github.com/microsoft/vscode-vsce/pull/532 and the reasonings for it to exist are specified at https://github.com/microsoft/vscode-vsce/issues/525.

Basically, I want to integrate ovsx with semantic-release through the existing plugin semantic-release-vsce (see https://github.com/felipecrs/semantic-release-vsce/issues/62).

The user will optionally supply an OVSX_PAT environment variable, and when supplied, the plugin will publish to both VS Marketplace and OpenVSX, making it very easy for existing adopters of the plugin to adopt OpenVSX as well.