Resolve the Windows PowerShell 5.1 compatibility contract

[MariusStorhaug]

Context and request

Install-GoogleFont contains a clean block, which Windows PowerShell 5.1 cannot parse. At the same time, the function retains a sequential-download branch intended for older PowerShell versions. The module README does not state an edition or minimum PowerShell-version requirement.

This leaves users with a parse failure instead of a clear compatibility outcome and leaves maintainers with contradictory support signals in the code.

Decision required

Choose and document one supported contract:

1. PowerShell Core only: add an explicit #Requires -PSEdition Core, document the requirement, and remove or justify compatibility paths that cannot be reached on Windows PowerShell 5.1.
2. Windows PowerShell 5.1 remains supported: replace unsupported syntax and validate both the sequential and PowerShell 7 parallel paths in CI.

Acceptance criteria

• The supported PowerShell editions and minimum version are explicit in source and README documentation.
• Unsupported hosts receive a clear, actionable error rather than a parser error.
• The code paths retained after the decision are covered by the supported-version test matrix.

Related

• Add #Requires -PSEdition Core directive to Install-NerdFont NerdFonts#62 tracks the analogous problem in NerdFonts.
• Extract shared download routine from parallel/sequential branches in Install-GoogleFont #214 notes the existing PowerShell 7 parallel and sequential-path split.