Add support for XDG base directory spec

[jrom99]

Currently, after opening the program, a directory named .asy is created in the home directory, hosting configuration and state files.

This request adds support for the XDG base directory spec https://wiki.archlinux.org/title/XDG_Base_Directory, which is a cross-platform specification that de-clutters the home directory and simplify backup management.

So, now instead of creating the ~/.asy folder, the code checks if it exist and then uses it, fallbacking to the appropriate XDG directory.

I took the liberty to simplify the code logic a bit, since there was also a possible bug in the code below, where the default options and current options were the same object, so the program wasn't actually reloading the settings when setDefaults was run.

asymptote/GUI/xasyOptions.py

Lines 22 to 30 in 205a13c

	def defaultOptions(self):
	if self._defaultOptions is None:
	f = io.open(self._defaultOptLocation)
	try:
	opt = cson.loads(f.read())
	finally:
	f.close()
	self._defaultOptions = opt
	return self._defaultOptions