doc: extensions: kconfig: allow to copy link to results

This patch allows copying a URL to the current Kconfig search results.
For example, I can now share the search results for "CONFIG_TEST.*". A
new button is added to the search box that, when clicked, copies to the
clipboard the URL.

This feature is achieved by prepending query hashes with '!'. When '!'
is found, the content of the URL hash is left intact, whereas before it
was always enclosed within '^$' to produce an exact match.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>