New theme token: --sl-input-required-content-color

[xdev1]

Some UI libraries like fluentui and antd use red asterisks for labels of mandatory input fields, which looks pretty nice. Shoelace on the other hand uses only black or white (depending on the theme) asterisks, which does not look that nice IMHO. Would be great if we could add new theme variable --sl-input-required-content-color to make this easily configurable. I personally would prefer the following default value:

--sl-input-required-content-color: var(--sl-color-danger-700);

... but if you prefer inherit it's also fine, of course :wink:

[claviska]

I'm fine with this addition since there's no other way to target the asterisk because it's generated in ::after.

[trailsnail]

Maybe that's a good issue for our Hacktoberfest event. We will have a look at this... Setup for contribution is already done and working 😁

[claviska]

Closing since this was added in #948.