Move the remaining call sites that constructed StringBuilder in UTF-16
mode over to Utf16StringBuilder or explicit UTF-8 to UTF-16 conversion.
Teach FormatBuilder to target Utf16StringBuilder directly so formatted
Utf16String construction no longer needs StringBuilder's UTF-16 mode.
Update the Utf16String tests to cover Utf16StringBuilder adoption and
clear behavior directly.
We currently have a single IPC to set clipboard data. We will also need
an IPC to retrieve that data from the UI. This defines system clipboard
data in LibWeb to handle this transfer, and adds the IPC to provide it.
