Add EditorConfig `.editorconfig` to project

[o-l-a-v]

Summary of the new feature

Add EditorConfig .editorconfig to the project, like other PowerShell related repos already have.

• https://github.com/PowerShell/PowerShell/blob/master/.editorconfig
• https://github.com/PowerShell/PSResourceGet/blob/master/.editorconfig
• https://github.com/PowerShell/vscode-powershell/blob/main/.editorconfig

Proposed technical implementation details (optional)

Unify style, syntax, rules, linting, formatting across PowerShell related projects.

What is the latest version of PSScriptAnalyzer at the point of writing

Not relevant.