wslvar: Allow variable names with parentheses

[lucyllewy]

In Windows on 64bit systems there is an environment variable named ProgramFiles(x86).

Description

Allow the use of wslvar to access ProgramFiles(x86) from Windows in WSL by wrapping the $env:* variable with curly braces {} so that parentheses are acceptible. This causes wslvar to access the PowerShell environment variables as (for example) ${env:ProgramFiles(x86)} preventing a failure.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read Code of Conduct and Contributing documentations.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.