Add concurrency to `mdformatter`

[saswatamcode]

This PR parallelizes the file processing loop in mdformatter for faster formatting and link checking using WaitGroup by spawning a goroutine for each file. (This also changes CLI spinner behavior as filenames cannot be shown due to multiple files being processed at once)

Some caveats,

• Panic during cancellation sometimes due to v.c.Wait()(flaky, maybe due to internal colly WaitGroup)
• Need some way to limit number of file processing goroutines

[bwplotka]

Still comments not addressed, I believe?

[bwplotka]

Any progress?