Align behavior with Primer guidance behind validate-after-first-blur toggle

[joelhawksley]

In the Primer Forms documentation, it states:

Don't attempt to validate an input before the user is done with it. Validation may be performed as the user is typing or making their selection, but only after the first time the input has been validated and the input is in an invalid state. This gives the user early positive feedback by removing the error if the user makes a change that fixes it.

However, auto-check-element currently validates on every input change.

I recommend that we update the behavior to match the Primer documentation behind a toggle that defaults to false, validate our use-cases in GitHub.com to see if they all support this recommended behavior, then decide whether to leave it as a toggle long-term or to make it the default behavior, potentially with or without a fallback to the old behavior, which would be a breaking change.