fix: Allow to use environment executable java to launch servers

[CsCherrYY]

fix #1249

This pr checks and keeps the consistent of the two servers (Gradle server and Gradle language server), and follow Gradle behavior.

We will check the following ones (with descent priority), Beside VS Code configuration, we use step 4-5 to keep consistent with Gradle itself behavior.

1. config "java.import.gradle.java.home"
2. config "java.jdt.ls.java.home"
3. config "java.home" (deprecated)
4. environment JAVA_HOME
5. environment executable java(.exe)

[Eskibear]

I don't see any async statements in the body.

[CsCherrYY]

Let me remove "async" of this function and getGradleServerEnv().

[Eskibear]

what will happen if undefined is returned? compared with previous behavior where env is returned.

[Eskibear]

prompt NO_JAVA_EXECUTABLE early before actually executing it?

[CsCherrYY]

prompt NO_JAVA_EXECUTABLE early before actually executing it?

Yes. Please refer: https://github.com/microsoft/vscode-gradle/pull/1263/files#diff-3369ade313508c278005bfe83330cb8fb4852a75783f5f5843d8d89a46423fc6R43-R46, if we can't find a proper java executable, will return early.

The previous behavior is always to run the start script via cmd variable, in this way, Gradle will report an error if no java executable can be found.

[Eskibear]

Just 2 cents, flip the if-else branch would be more human-readable for me, because you don't assign a new javaHome in if !javaHome block..

Below makes more sense to me:

a)

if (!javaHome) {
    javaHome = .....
    // do something requiring javaHome
} else {
    // do something requiring javaHome
}

b)

if (javaHome) {
    // do something requiring javaHome
} else {
    // do something unrelated with javaHome
}