Add pwsh alias for “PowerShell” lexers

[ExE-Boss]

This matches the official executable name of PowerShell 6 and newer:

• Create pwsh as a shorter name for calling powershell.exe PowerShell/PowerShell#4214

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Also, the posh alias should be deprecated as it clashes with the “Policy‑compliant Ordinary SHell”.

[birkenfeld]

Please run make mapfiles and include the change in the PR.

[birkenfeld]

As for posh, since that isn't something we'll have a lexer for, let's keep the alias.

[Anteru]

Merged, thanks a lot!