Fix layout issue in Shortcode

[chipsnyder]

Fixes #1868
Full description in gutenberg WordPress/gutenberg#20097

To test:
1.) Select a shortcode block
2.) Type enough text to wrap to a second line or more

Expect:

• Text input to grow with content
• The device should not automatically force an autosuggest change.
• Some devices/APIs won't show the prompt at all.

3.) Delete text
Expect:

• Text input to shrink with content

4.) Paste content
Expect:

• Text input to grow with content
• The text should have similar wrapping as a prefilled block.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[geriux]

LGTM! Approved by WordPress/gutenberg#20097 (review)