New ELPA packages: gnat-compiler, gpr-query, gpr-mode

[Stephen Leake]

I've split out three packages from ada-mode, to make it easier to manage
eglot as a possible backend, and to simplify building the corresponding
Ada code in Alire:

gnat-compiler - Support for running GNAT tools

gpr-query - provides an xref backend using cross reference information
output by the GNAT compiler

gpr-mode - major mode for gpr files (GNAT project definitions)

Ok to commit to ELPA?

-- 
-- Stephe