implement prototype for using new javac-based JDT parser

[martinlippert]

The majority of the information around the incubated version of JDT that allows you to switch between javac and ejc-based parsing of Java source code can be found here: https://github.com/redhat-developer/vscode-java/releases/tag/v1.36.0

The idea here is to build a prototype of the spring boot language server that uses this incubating version of JDT with javac-based parsing enabled. Questions would be:

• what do we need to change in order to adopt this?
  • parsing source code (bulk)
  • parsing source code (single file)
  • caching the environment (to speed-up lookup when re-parsing a file)
• what are the performance implications of this? (running sample projects of up to 60k source files)
• what are the memory implications of this? (running sample projects of up to 60k source files)