[ShaderGraph] [2021.2] Fix "view shader" commands to not block Unity from closing, use same editor instance

[cdxntchou]

Purpose of this PR

Backports:
2022.1: #5735
2021.2: #5736

Fix for https://fogbugz.unity3d.com/f/cases/1367188/

Previously, when viewing the generated shader output via the debug commands in ShaderGraph, a separate instance of the script editor would be invoked for each generated shader, instead of re-using the existing editor.

Worse, when the user tried to close Unity, it would block on an inscrutable dialog:

Until the user closed all of the script editors that had been opened.

This PR fixes both issues by switching to use the script function: "OpenFileAtLineExternal" to invoke the script editor, instead of directly creating editor processes.

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Testing status

• Tested opening generated shaders via the ShaderGraph Inspector (both buttons)
• When using Visual Studio, validated that they open in the existing VS instance
• When VS is not running, it opens a new instance of VS.
• Validated that changing the "script editor" preferences is reflected in what editor is opened.
• Tested right click on node -> show generated code -- all of above works.
• Tested right click on node -> show preview code -- all of above works.

Yamato:

PR ShaderGraph: 🟢
https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/2021.2%252Fsg%252Ffix%252F1367188/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2/8870350/job/pipeline

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Comments to reviewers

Notes for the reviewers you have assigned.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
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[jessebarker]

LGTM

[marctem]

Works fine with Rider on Mac