rename `importmap` to `importMap`

[nnmrts]

Context: For the last few days I'm trying to make denon and https://github.com/jurassiscripts/velociraptor script files compatible, in the sense that one can use one file for both tools. I've opened several PRs and discussions at the vr repo and now annoying me is arriving here. :P

While creating jurassiscripts/velociraptor#100 I even mentioned denon using importmap. In my opinion, both should switch to importMap though, because Deno uses importMap, or more accurately, --import-map for its CLI. Until recently, I thought the Deno CLI arg was --importmap, and in fact the CLI still supports that, but with version 1.5.0 it was renamed to --import-map: denoland/deno#7032.

I'll see how complicated this gets and will probably create a PR for this later.

Also pinging @umbopepato here, in case they are interested.