Add "disabled" flag for forms

[djmattyg007]

This function solves the problem (这个功能解决的问题)

When a form is submitted, it is a good idea to disable all inputs to prevent the user from making changes that won't take effect. Right now, this must be done for each form field individually.

Expected API (期望的 API)

<n-form> should have a disabled prop that will automatically disable all fields inside that form when set to true.

[07akioni]

I think it's a reasonable idea.