[KTextbox]: Allow label to be passed via slot

[MisRob]

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Current behavior

The only way to pass a label is via the label prop.

Desired behavior

• Label can be passed either via label prop or label slot
• There is validation that throws a warning in case a label is not passed via one of them
• There is a documentation on how to use and not use the label slot

Value add

• Supports use-cases when displaying a label visually is not desired (e.g. token readonly input in Studio's channel collections table)

<KTextbox>
  <template #label>
    <span class="visuallyhidden">Label</span> 
  </template>
</KTextbox>

• Will allow to get rid of implementations when visual requirements led to label being bypassed by label=" ", such as here and other places, resulting in poor a11y due to missing labels.

• Allows for any future customization of labels, should the need arise

Possible tradeoffs

General tradeoffs related to allowing anything be passed. Documentation will have guidance though.