Updates build.ps1 with ArgumentCompleter attribute on Task parameter

[webtroter]

This adds autocomplete to the Task parameter.

Description

The ArgumentCompleter attribute works by reading the psakefile.ps1 with Get-PSakeScriptTasks and providing autocomplete suggestions.

Related Issue

Discussion #25

Motivation and Context

Helps Task discovery

How Has This Been Tested?

Works on my machine, and seems simple enough

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[webtroter]

I think I don't have tests to add since the change is part of the template. Is that correct?

[devblackops]

Thanks for the contribution @webtroter! This is a good addition.