feat: allow java.jdt.ls.java.home to use execution environment names

[amritamishra01]

Summary

This change allows java.jdt.ls.java.home to accept the name of a configured Java execution environment from java.configuration.runtimes, in addition to accepting an absolute JDK path.

Motivation

Currently, java.jdt.ls.java.home expects a filesystem path. If users specify the execution environment name (for example, "JavaSE-21"), the value is treated as a path and validation fails.

This change resolves the configured runtime name to its corresponding JDK path before the existing validation logic runs.

Changes

• Added runtime-name resolution in checkJavaPreferences().
• Preserved existing behavior for absolute JDK paths.
• Left the validation logic in requirements.ts unchanged.

Testing

• Verified that an absolute JDK path continues to work as before.
• Verified that a configured runtime name (for example, "JavaSE-21") is resolved successfully and the Java language server starts normally.