Template part area: Add internationalised plural form

[jameskoster]

When you replace a template part like a header or a footer, the following modal appears:

The modal could also soon appear when you insert a Header or Footer block via the Inserter: #42142.

It would be nice if we updated the 'Existing template parts' header to make use of the semantic area so that it reads 'Existing headers' or 'Existing footers'.