mx: improve first usage experience: Create mx.ask_persist_env to handle env file modifications, make it deal with files missing final newline Use mx.ask_persist_env to persist the DEFAULT_VM Make mx_graal specify its version restrictions when asking for the default JDK When selecting versions manually or automatically, use JDKs from JAVA_HOME first, then from EXTRA_JAVA_HOMES, then from standard locations, then sort by version. Even if EXTRA_JAVA_HOMES is already defained, let the user decide which JAVA_HOME to use

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