Add syntax highlighting for 'clean {}' block

[daxian-dbw]

This is a follow-up PR of #186, because of the renaming of the new named block.

function Get-Foo {
    [CmdletBinding()]
    param()

    begin { }
    process { }
    end { }
    clean { }
}

Reference https://github.com/PowerShell/PowerShell/pull/15177 for indication on when the feature will become available. https://github.com/PowerShell/PowerShell/pull/15177 did some design changes based on the original PR PowerShell/PowerShell#9900

/cc @andschwa and @vexx32

[andyleejordan]

Thanks for this @daxian-dbw! We'll get it merged closer to the timeline of the feature itself being merged.

[daxian-dbw]

@andschwa https://github.com/PowerShell/PowerShell/pull/15177 has been merged, so can you please review this PR and maybe merge it as well?