src: fix -Wmaybe-uninitialized compiler warning

Turn the `strategy_` method pointer into an enum-based static dispatch.

It's both safer and more secure (no chance of method pointer corruption)
and it helps GCC see that the shift and suffix tables it's complaining
about are unused in single char search mode.

Fixes the following warning:

    ../src/string_search.h:113:30: warning:
    ‘search’ may be used uninitialized in this function [-Wmaybe-uninitialized]
         return (this->*strategy_)(subject, index);

Fixes: https://github.com/nodejs/node/issues/26733
Refs: https://github.com/nodejs/node/pull/31532
Refs: https://github.com/nodejs/node/pull/31798
PR-URL: https://github.com/nodejs/node/pull/31809


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>