do not exclude markdown when detecting comment for Delegate codelens

[joshspicer]

Before

After

[joshspicer]

cc/ @alexr00 I am not sure why isComment excludes markdown (forcing it to return true) in the first place. I did this change to avoid a behavior change in other places, but may want to change the default behavior of isComment?

[alexr00]

It's because we always want to show the issue completion provider and issue link provider (which is actually disabled) in markdown. But, this can be moved into the issue completion provider.