Add-import code action

[georgewfraser]

Currently, JLS tries to automatically add imports on save. This works well for scenarios like copy-paste, but sometimes the correct import is ambiguous and it does nothing. Implementation is here:

java-language-server/src/main/java/org/javacs/CompileBatch.java

Line 291 in 65ca5e4

public List<String> fixImports(URI uri) {

We should have a code action to add imports, like TypeScript.

#44

@albfan