feat: Add set-default option (#1017)

* Add set-default option

This option allows to install an additional JDK without making it the
default one.

I have wanted this for quite a long time as I'm running custom GitHub
Actions with Java, which might require a specific JDK and I don't want
to pollute the JDK that is used by the overall workflow calling the
action.
And I'm apparently not alone as there was a preexisting issue.

Fixes #560

* Dedupe setJavaDefault and document multi-version/toolchain behavior

- Refactor setJavaDefault to delegate shared output/env logic to
  setJavaEnvironment, avoiding duplication between the two.
- Document that set-default applies to all JDKs in a multiline
  java-version, and that installed JDKs remain registered in Maven
  toolchains regardless of set-default.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* test: fix Prettier formatting in base-installer test

Resolves the failing 'Basic validation / build' format-check on
__tests__/distributors/base-installer.test.ts (line exceeded print width).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Bruno Borges <brborges@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/distributions/local/installer.ts

@@ -69,9 +69,15 @@ export class LocalDistribution extends JavaBase {
       foundJava.path = macOSPostfixPath;
     }
 
-    core.info(`Setting Java ${foundJava.version} as default`);
-
-    this.setJavaDefault(foundJava.version, foundJava.path);
+    if (this.setDefault) {
+      core.info(`Setting Java ${foundJava.version} as the default`);
+      this.setJavaDefault(foundJava.version, foundJava.path);
+    } else {
+      core.info(
+        `Installing Java ${foundJava.version} (not setting as default)`
+      );
+      this.setJavaEnvironment(foundJava.version, foundJava.path);
+    }
     return foundJava;
   }