feat: requestedJavaVersion honored by init templates

[maxandersen]

alternative/replacement of #2143

in #2143 it was suggested to force add "//JAVA 25+" when compact source enabled (automatically enabled when Java > 25) but I turned it down as it was resulting in inconsistent behavior.

Since then I myself been caught by having done jbang init --java 25 hello.java and then when running ./hello.java it didn't work because my environments java was less than Java 25.

so this PR suggests introducing a requestedJavaVersion property which will be set to null if no specific java version requested; and other set to the raw string of the arg value in --java <arg>

Then use that to generate //JAVA ${requestedJavaVersion} if requested version is present, and in the case of compact source files will get //JAVA 25+ if you did not request a specific java file...meaning no matter how you get a compact source file - it will always have at least //JAVA 25+ in it OR what you requested using --java <arg>

In addition I updated the compact source to use IO.println() to be able to cut out the import line.

///usr/bin/env jbang "$0" "$@" ; exit $?
//JAVA 25+

void main(String... args) {
    IO.println("Hello World");
}

I also added {#if requestedJavaVersion} //JAVA {requestedJavaVersion} {/if}

to the templates I could see it being meaningful.

wdyt @wfouche and @quintesse - I think this should make us all more happy than annoyed ;)

[maxandersen]

btw. yes - this is up to the individual templates wether they want to honor this or not. but we already have templates setting specific java versions so i dont think there is much way around this.

[wfouche]

I like this change, and it is future proof. :-)

[quintesse]

I think it's a good compromise. Merge it! :-)