With https://openjdk.java.net/jeps/400 implemented in Java 18,
"file.encoding" system property became meaningless and can't be used
anymore to determine system native encoding.

Instead, use new Platform.getSystemCharset() API that tries to provide
a suitable replacement compatible with Java 18+ and previous Java
versions.

Note: the proposed change makes sure that IF users have specified
encoding via command line arguments -Dfile.encoding=XYZ, it will be
still used.

This PR fixes
https://github.com/eclipse-platform/eclipse.platform.resources/issues/154