Add gprbuild syntax

GPRbuild is an advanced build system designed to help automate the construction of multi-language systems. This PR adds support for the GPR configuration file syntax that is used by the tool. The syntax is loosely based on Ada, with very similar syntax, but completely different semantics (obviously, since it has completely different goals as a language)

Project at https://github.com/AdaCore/gprbuild

User Guide at https://docs.adacore.com/gprbuild-docs/html/gprbuild_ug/gnat_project_manager.html#introduction