JS: Re-add optimization for literal strings in genApp (fixes #23479)

Based on !10588



Co-authored-by: Sylvain Henry <sylvain@haskus.fr>
Co-authored-by: Andrei Borzenkov <root@sandwitch.dev>
Co-authored-by: Danil Berestov <goosedb@yandex.ru>

-------------------------
Metric Decrease:
    T25046_perf_size_gzip
    size_hello_artifact
    size_hello_artifact_gzip
    size_hello_unicode
    size_hello_unicode_gzip
-------------------------