Fix deprecated trailing semicolons in macros

[notriddle]

This silences a very noisy deprecation warning from rustc:

warning: trailing semicolon in macro used in expression position
   --> src/webcore/macros.rs:179:141
    |
179 |   ... [$($arg_names)* $arg_name] [$($unused_arg_names)*] -> $($rest)* );
    |                                                                        ^
    |
   ::: src/webapi/form_data.rs:144:9
    |
144 | /         js! (
145 | |             return @{self}.has(@{name});
146 | |         ).try_into().unwrap()
    | |_________- in this macro invocation
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
    = note: this warning originates in the macro `$crate::_js_impl` (in Nightly builds, run with -Z macro-backtrace for more info)