Fix split() to dispatch to base::split() for non-data-frame input

[ielbadisy]

Summary

• split() unconditionally treated its first argument as a table, breaking the base R idiom split(vector, factor) for any non-data-frame input, including inside R Markdown/Quarto's own rendering pipeline (which calls split() unqualified internally)
• split() now checks is.data.frame() and dispatches to base::split() when the input is not a table, keeping table-splitting behavior unchanged
• Bumps to 1.4.2, updates NEWS.md and cran-comments.md for resubmission

Test plan

• devtools::test() -- 783 pass, 0 fail
• R CMD check --as-cran -- 0 errors, 0 warnings, 3 benign NOTEs (days-since-update, local compiler flag, sandbox clock)
• Manually verified split(1:10, rep(1:2,5)) matches base::split(), and split(iris, by="Species") still returns basetable pieces