build: i18n: WIP: check exepath for ICU data

==WORK IN PROGRESS==

If the user hasn't otherwise specified a directory,
append `NODE_EXEPATH_ICUDIR` ( default: ../share/node/icu/ )
and set it as the ICU dir.

If we are in `small-icu` mode, ALSO actually verify the aforementioned
directory exists (thx libuv) and suppress loading the small-icu data
in that case.

(If we aren't in `small-icu` mode, there's no reason to incur the cost
of hitting the FS to check whether the directory exists or not at
startup time.)

// cc: @tjfontaine