Use Git submodule for PowerShellEditorServices dependency

[andyleejordan]

PR #3286 has convinced me we need a better solution to bundling our PowerShellEditorServices dependency, because the current approach of relying on an ADO pipeline to download the correct bits isn't great (obviously). It also makes it next to impossible to reliably determine from the Git sources of the extension which version of the LSP server it's meant to interact with.

[andyleejordan]

FYI @rjmholt

[andyleejordan]

Sure, you can check the changelog, but that means we (humans) have to get it right, and there's no real way to automate testing of this because there's nothing in this extension which actually states which version of the dependency it uses. Issue #3282 could really only have been avoided by another manual check (which I'm adding to my release process notes, but still).

[andyleejordan]

So I believe that I've essentially resolved this but without resorting to submodules, with two things:

PowerShell/PowerShellEditorServices#1476 ensures that given a build of PSES, we know exactly which commit it came from, and this is available to the extension too (indeed, it's printed at startup!):

== Build Details ==
- Editor Services version: 2.4.6
- Build origin:            Release
- Build commit:            e03e50547b2c90820e94b18bace2229e54d917f0
- Build time:              7/14/2021 4:46:01 PM

And a987d4d ensures that given a CI/CD run of the extension, we also know (very easily) which commit of PSES was picked up (in the case of CI) or which build of PSES was picked up (in the case of CD), all available under the run's consumed artifacts: