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Add shellPath to allow user specify preferred shell. Fix #10917

Fixes #9448

This change needs to be made at this layer since a change to URI.fsPath
(the root cause) is too risky as lower case drives are relied upon.

Part of #9448

Fixes #11714

Revert "fix behaviour with multiline vs singleline snippets and their final cursor position, #11742"

This reverts commit 46c86891.