Improve member completion

[MartinGC94]

PR Summary

Fixes #10529
The member completion code wrongly assumed that the parent to the error statement was what should be completed, instead of traversing up to where the cursor actually was. Now you should get accurate member completion regardless of where the cursor is.
This also adds member completion for switch statements like:

switch ($x)
{
    ''.<Tab>
    {
    }
}

Allows completion of members/attribute arguments regardless of surrounding spaces/new lines (If the parser would allow it in that context).
Adds a test for a seemingly untested scenario: ls "".<Tab>
And filters out attribute arguments that have already been used in the attribute, for example:

function Verb-Noun
{
    Param
    (
        [Parameter(Position=0, <Tab>)]
        [string[]]
        $Param1
    )
    process{}
}

will not show Position again.

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[ghost]

This pull request has been automatically marked as Review Needed because it has been there has not been any activity for 7 days.
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[iSazonov]

Formally LGTM but I don't understand why we need cases like '[System.Management.Automation.ActionPreference]::Break. ^'. It's up to other reviewers.

[MartinGC94]

Formally LGTM but I don't understand why we need cases like '[System.Management.Automation.ActionPreference]::Break. ^'. It's up to other reviewers.

For me it's just for the sake of consistency. If the parser allows it then I don't see why the completion code shouldn't support it either.

[PaulHigin]

@TravisEz13 Can you please review?

[ghost]

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Maintainer, please provide feedback and/or mark it as Waiting on Author

[PaulHigin]

@daxian-dbw Can you please review?

[ghost]

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[pull-request-quantifier-deprecated]

This PR has 222 quantified lines of changes. In general, a change size of upto 200 lines is ideal for the best PR experience!

---

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Label      : Large
Size       : +184 -38
Percentile : 62.2%

Total files changed: 3

Change summary by file extension:
.cs : +94 -37
.ps1 : +90 -1

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[daxian-dbw]

LGTM. Thanks @MartinGC94!

[ghost]

🎉v7.3.0-preview.4 has been released which incorporates this pull request.:tada:

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