This PR refactors the building scripts and fix the AppVeyor CI. Major changes are:
1. Add `build.ps1` to provide simple entry points that can be used in CI.
2. Add `tools/helper.psm1` to group utility functions. Move the scripts for `RunTests` to `helper.psm1`. The plan is to move big chunk of build action scripts to the `helper.psm1` to make `PSReadLine.build.ps1` less cumbersome but more clear about what build actions are available.
3. Fix the AppVeyor CI.