field-level validation in settings

in plugin.json > settings, define an (optional) validation function that will get called for each field in the settings, with a signature like:

onConfigFieldValidate(fieldName: string, fieldValue: string | number | ... ): string

The validation function will return an empty string '' if validation passes, or an error message to be displayed (e.g. in red under the field) if it does not pass.

The only tricky bit is when to call the validation function. Feels like the ideal is to do all of:
a) on settings load (when the settings window is brought up)
b) on blur (when someone has entered and left a field)
c) on "Save and close" click (errors would cancel the close)

More discussion here:
https://discord.com/channels/@me/928678751808671865/935560749823164486