Include gir files in GirTool as embedded resources

[adamreeve]

Related to #1078

In order to make GirTool useful as a standalone tool, it would be helpful to have the gir files for the common GObject based libraries already supported by gir.core available as embedded resources, so that users don't need to check out the gir-files repository to use GirTool.

Users would then need to ensure that they reference the required GirCore NuGet packages corresponding to the dependencies needed by their library, and the package versions should match the version of GirTool used.

I don't think this needs a new command line argument to GirTool, but the logic for looking up a gir file would be to first check in the directories from the --search-path-linux/macos/windows arguments, and if a file isn't found there, look it up from the embedded files.