Add environment for generator scripts

[Vexatos]

Minimal Project file for the generator scripts. See https://github.com/JuliaGraphics/Gtk.jl/pull/535#issuecomment-760067299.

From CI or tests, it'd be called like this:

using Gtk # latest Gtk from dev, imported in the main or test Gtk env
using Pkg
Pkg.activate("$root/gen")
Pkg.instantiate()
include("gtk_auto_gen.jl")

The generator scripts depend on the dev version of Gtk.jl, but actually making the environment depend on it isn't possible without including the Manifest.toml (using repo-url = ".."), which probably shouldn't be done.

[tknopp]

But aren't the tests run in dev mode? Not sure how this is accomplished in the test's project file

[Vexatos]

Testing in julia using Pkg.test() is a kind of special environment, so yes, tests are run using Gtk from the directory, meaning if you were to run the generator script from inside the test environment (not that I'd recommend that) you'd get the correct output. Similarly, you can run it starting from the primary Project.toml in any CI step for the same effect. Gtk.jl doesn't have a separate Project.toml for the tests yet so I think it does that anyway.

[codecov[bot]]

Codecov Report

Merging #555 (c128488) into master (8742856) will increase coverage by 0.08%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   47.43%   47.52%   +0.08%     
==========================================
  Files          32       32              
  Lines        2382     2382              
==========================================
+ Hits         1130     1132       +2     
+ Misses       1252     1250       -2     

Impacted Files	Coverage Δ
src/GLib/signals.jl	73.97% <0.00%> (+0.91%)	:arrow_up:

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