add os.getDataDir for platform independent XDG_DATA_HOME

[timotheecour]

(analog to getTempDir and getConfigDir in std/os)

refs nim-lang#18119 (comment)

links

• add os.getCacheDir nim-lang/Nim#18126 (which is different: getCacheDir)

[juancarlospaco]

Would it be better to make something like
func getXdg*(items = {XdgHome, XdgData, XdgTemp})

Because theres multiple paths in XDG_*; Then the pre-existent proc can be a
template getHome*() = getXdg({XdgHome}).

[timotheecour]

but XDG doesn't translate to windows; that's why we're going for these instead:
getHomeDir
getTempDir
getConfigDir
getCacheDir
getDataDir (no PR yet for this)

[juancarlospaco]

Are you sure ?. 🤔