Support packages with only one version

[silvervest]

Packages with only one version will return a flat string rather than an array via npm view, so this supports that

Associated error may look like this:

Fetching information for 31 packages...
 ████░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░ 10% | ETA: 30s | 3/31undefined:1
10.2.0
    ^

SyntaxError: Unexpected non-whitespace character after JSON at position 4 (line 1 column 5)
    at JSON.parse (<anonymous>)
    at getVersionsForPackage (/opt/homebrew/lib/node_modules/pkgmigr8or/index.js:56:15)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async /opt/homebrew/lib/node_modules/pkgmigr8or/index.js:25:15
    at async /opt/homebrew/lib/node_modules/pkgmigr8or/index.js:35:24
    at async Promise.all (index 9)
    at async /opt/homebrew/lib/node_modules/pkgmigr8or/index.js:77:31

Cheers!