RFR: Generate Java enums

[Vivek Narang]

We are using jextract in our [cuVS Java](https://github.com/rapidsai/cuvs/tree/branch-25.08/java) project. As of now, jextract generates individual getter functions for the C enums. This results in code where it is difficult to disambiguate which getter functions are logically grouped together for a particular Enum. Additionally, given the lack of this capability, we also have to manually create and maintain Java enums for each enum in the C layer.

In this PR, I wish to introduce an option to additionally generate Java enums by passing an optional flag `--generate-java-enums`. In case a Java user wishes to use bitwise operations on the enums, they can use the enum's `.ordinal()` method for this. The original static getter methods will still be generated as before. 

Example C enum:

enum SIZE {
   S,
   M,
   L
};

Produces the following with jextract:

private static final int S = (int)0L;
public static int S() {
     return S;
}

private static final int M = (int)1L;
public static int M() {
     return M;
}

private static final int L = (int)2L;
public static int L() {
     return L;
}

With this feature enabled using the `--generate-java-enums` flag, the following enum is generated (in addition to the above):

public enum Size {
    S(0),
    M(1),
    L(2);

    private final int value;

    private Size(int value) {;
        this.value = value;
    }

    public int getValue() {
        return this.value;
    }
}

This PR is created against the `jdk22` branch. I will rebase it to `master` branch if needed.

-------------

Commit messages:
 - Remove extra space in messages.properties
 - Generate enums in header file instead
 - Initial work

Changes: https://git.openjdk.org/jextract/pull/284/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=284&range=00
  Stats: 184 lines in 12 files changed: 168 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jextract/pull/284.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/284/head:pull/284

PR: https://git.openjdk.org/jextract/pull/284