fix: replace ssh config block in-place

[mafredri]

This PR introduces two changes:

1. Replace vscode block in-place in the SSH config
2. Adds an additional guard against replacing user-added blocks that look like an old-form SSH config block (without block markings)

TBH, we could probably remove the old block detection entirely instead of this additional check, but didn't want to make that decision here.

[ericpaulsen]

related issue: #210