Emit a guard newline in TextareaElement to preserve leading newlines

A browser's HTML parser ignores a single newline immediately after a
<textarea> start tag, so a value whose first character is a

Emit a guard newline in TextareaElement to preserve leading newlines

A browser's HTML parser ignores a single newline immediately after a
<textarea> start tag, so a value whose first character is a newline lost
it on round-trip (e.g. saving a page or section whose source starts with
a blank line silently dropped that line).

Emit a guard newline right after the start tag so the one the browser
drops is absorbed and the value stays intact.

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