VSCode: Add Scarb class and move Scarb's LS discovery there

[mkaput]

Stack:

• VSCode: Move isScarbProject logic to separate file #5015
• VSCode: Slightly simplify LS selection logic #5014
• VSCode: Extract standalone CairoLS logic to StandaloneLS class #5013
• VSCode: Move Scarb searching logic to the Scarb class #5012
• VSCode: Encapsulate all configuration magic into Config class #4976
• VSCode: Move away and simplify tool finding functions #4975
• VSCode: Add Scarb class and move Scarb's LS discovery there #4974 ⬅
• VSCode: Do not spawn LS process manually and use VSC idioms instead #4973

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

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This change is 

[mkaput]

Closing pull request: commit has gone away

[orizi]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @mkaput)