Make locale parameter case insensitive

[kathy-phet]

We had discussed removing case sensitivity in our locale checking so
xx and xX and XX are all equivalent
as are
xx_XX or XX_xx or xx_xx

We will still recommend and communicate the standard casing, but will be a bit relaxed in what we accept since case sensitivity does not seem to be important here.

[kathy-phet]

phetsims/qa#1085