Improve variable completion performance

[MartinGC94]

PR Summary

This is a small rewrite of the variable completion code to make it faster.
While the general performance was fine, the first variable tab completion was too slow for me.
On my PC the initial variable completion inside a long script takes about 1.8 seconds, and subsequent attempts take 50ms.
With the new code the first attempt takes about 100ms, and drops down to around 4ms for subsequent attempts.

The logic stays mostly the same, I've simply replaced Get-Item calls with the equivalent internal API calls to skip the PS invocation overhead. Most of the performance improvements however come from the updated Astvisitor code. Instead of adding every variable instance to a list and later analyzing the list with a bunch of duplicates, I only look for variable assignments and save the most recent type constraint and static assignment types in a dictionary for later reference.

There are some minor functional changes that are worth discussing:

1. When analyzing script text I don't process variables after the cursor. This seems more accurate but maybe users like having all the script variables show up in the completion, even if they appear after the cursor.

2. For commands with "OutVariable" I don't bother using the type inference to try and figure out the output type of the command and instead assume it's an arraylist. The reason for this is that PowerShell doesn't unwrap the ArrayLists created by these parameters so it's technically more accurate to list those variables as ArrayLists.

3. Pipeline variables are only included in the completion result if the cursor is within the pipeline.

PR Context

Like I said, I thought it was a bit too slow.
Also:
Fixes #18688

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    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
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[daxian-dbw]

BTW, I agree with the 3 functional changes you called out in the PR description. I think they make sense.

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This PR has 361 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       : +239 -122
Percentile : 76.1%

Total files changed: 2

Change summary by file extension:
.cs : +222 -122
.ps1 : +17 -0

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[daxian-dbw]

LGTM

[daxian-dbw]

@MartinGC94, thanks for your contribution!

[ghost]

🎉v7.4.0-preview.4 has been released which incorporates this pull request.:tada:

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