fix: Show the hints in Java Projects explorer properly

[jdneo]

The PR is to let the Java Project explorer properly show hints for users:

• When redhat.java is not installed, hint for installation
• When no Java projects found, hint for creation/opening
• When JDT.LS in Lightweight mdoe, hint for switching.

Now the activation events is the same as redhat.java. But since events like workspaceContains is not supported in when clause, so I use APIs to query files during activation and set context value to imitate the condition:

• resourceLangId == java is for onLanguage:java
• java:workspaceContainsBuildFiles -> workspaceContains
• java:serverMode is to make sure the explorer will always visible once JDT.LS is activated. (still visible when user switch focus to a non-Java file)

fix #551

[CsCherrYY]

After testing, I find no regression but an existing bug about export jar.

When there is no workspace and the language server has been activated, the export jar button doesn't show in the Java Projects view title. However, the user can trigger export jar via command palette Java: Export Jar and get the following error.

I'd like to make the when clause of java.view.package.exportJar in command palette be alignment with that in view title to fix this bug. @jdneo Could you please add a commit about this?

[Eskibear]

LGTM