Add append to PATH environment modification #22
Description
From https://gitlab.com/dkml/distributions/dkml/-/issues/31:
I saw that PATH had been modified with the path to Dkml in front, which is not a recommended practice
That was for DkML but it is pertinent to dk (the successor to DkML). The EnvMod for MlFront_Core, including the new command language in MlFront_Thunk, only have an option to prepend to the PATH. Prepending is correct when spawning, but appending is correct when installing. So both prepend and append need to be supported (or have a generic xpend that does prepend when spawning and append when installing).