feat(text-editor): add placeholder

[LawrenceBorst]

Fixes Lundalogik/crm-feature#4039 Fixes Lundalogik/crm-feature#4074

Review:

• [ ] Commits are atomic
• [ ] Commits have the correct type for the changes made
• [ ] Commits with breaking changes are marked as such

Browsers tested:

(Check any that applies, it's ok to leave boxes unchecked if testing something didn't seem relevant.)

Windows:

• [ ] Chrome
• [ ] Edge
• [ ] Firefox

Linux:

• [ ] Chrome
• [ ] Firefox

macOS:

• [ ] Chrome
• [ ] Firefox
• [ ] Safari

Mobile:

• [ ] Chrome on Android
• [ ] iOS

[github-actions[bot]]

Documentation has been published to https://lundalogik.github.io/lime-elements/versions/PR-2907/