Add check for MacOS Sonoma running R < 4.3.2

[AlexAxthelm]

In MacOS Sonoma (Darwin version 23), iconv() can cause an R-fatal error when converting non-ASCII characters to ASCII.

See https://groups.google.com/g/r-sig-mac/c/pFuJu7F33Eg/m/xdrBmg-PDwAJ for more discussion.
Closes #54

[cjyetman]

pacta.portfolio.import/R/zzz.R

Lines 15 to 22 in 6125ae6

	"Warning: In Darwin release 23.0.0 (MacOS Sonoma 14.0) or higher,\n",
	"an issue with `iconv` causes R to crash when converting ",
	"Non-ASCII characters to ASCII.\n",
	"You are running R version ", r_version,
	" on Darwin release ", darwin_version, ".\n",
	"Please ensure that portfolios imported with ", pkgname,
	" contain only ASCII characters,\n",
	"or update R to version 4.3.2 or higher.\n"

This is neither entirely correct nor specifically scoped to the actual problem, as far as I understand.

The problem is specific to using the sub argument of iconv on a string that contains a " followed by an extended character on <= R v4.3.1 and macOS Sonoma (Darwin version 23) or higher

[AlexAxthelm]

Closing in favor of #58 which solves the actual problem.