Support placeholder for textbox property editor UI

[bjarnef]

Sometimes it would be useful to fill an optional placeholder value in configuration of textbox (perhaps also textarea) to guide content editors and the expected value/format of input field.

We can use description of property, but is often makes description even longer and a placeholder feels more in context.
Another option is creating a custom property editor UI, but seems a bit overkill.

E.g. for an reference number, license key or similar: WEB-XXX-XXX-XXX

This would be a simple solution.

In future the Umbraco UI library could possible support masking for input components, which also restrict characters to enter, e.g. numeric only, while filling separators:
https://ui.nuxt.com/docs/components/input#with-mask
https://primevue.org/inputmask/