Thanks to @trflynn89 for the neat implicit consteval ctor trick!
This allows us to basically slap `CheckedFormatString` on any
formatting function, and have its format argument checked at compiletime.
Note that there is a validator bug where it doesn't parse inner replaced
fields like `{:~>{}}` correctly (what should be 'left align with next
argument as size' is parsed as `{:~>{` following a literal closing
brace), so the compiletime checks are disabled on these temporarily by
forcing them to be StringViews.
This commit also removes the now unused `AK::StringLiteral` type (which
was introduced for use with NTTP strings).
This patch makes `Shell::block_on_job()` pump the event loop while the
job it's waiting for hasn't finished.
As this no longer pushes new event loops, it has the effect of
flattening the stack as well.
Fixes#4976.
