Setup `codeql-development-toolkit` repo for Copilot compatibility

[data-douser]

We should be making best use of GitHub Copilot in maintaining this codeql-development-toolkit repository.
This issue is about setting up the project files that will help us optimize the results when assigning this repository's GitHub Issues to the Copilot Coding Agent.

Specifically, the goals of this issue are to:

1. Setup the appropriate .github/instructions/*.instructions.md files to provide minimal, custom instructions for the most important and/or commonly edited files in this repository (i.e. especially for src/**/*.cs and src/**/*.liquid files. These custom instructions files should follow the guidance from writing-effective-repository-custom-instructions and using-one-or-more-instructionsmd-files.

2. Setup the copilot-setup-steps job / workflow to pre-install dependencies used by the Copilot Coding Agent, in accordance with the preinstalling-tools-or-dependencies-in-copilot-environment documentation. The existing .github/workflows/*.md actions workflows and other project build scripts should be consulted when determining what dependencies must be pre-installed for Copilot to be effective as an development agent for this repo. The most important dependency is that dotnet 6.0.0 must be installed for the qlt (aka CodeQLToolkit.Core) CLI to be runnable, which is important for any number of development and testing tasks for this repo. The check(s) for the new "Copilot Setup Steps" workflow should be passing for the PR that you submit for this Issue.

[data-douser]

Instead of a single .github/copilot-instructions.md file, this repo should have multiple *.instructions.md files in a new .github/instructions/ directory. This aligns with the guidance from using-one-or-more-instructionsmd-files, where each .github/instructions/*.instructions.md file should have a frontmatter section like:

applyTo: "some/repoRelative/globPattern/**"
description: "Short, actionable description of the actions file"

That approach should be improved in a revision to PR #31 .