Allow `yamlValidation` to be defined programmatically

[milesj]

Is your enhancement related to a problem? Please describe.

Nope.

Describe the solution you would like

The contributes.yamlValidation functionality works great, but we want to use versioned schemas without having to continuously update the extension.

Since package.json is static, these schema URLs are also static.

It would be nice, if possible, to set yamlValidation programmatically through the JS extension.

Describe alternatives you have considered

N/A

Additional context

N/A