fix serveStatic method

[hironichu]

Description

This PR fixes a small issue running Danet on windows using the serveStatic method.

On windows when we use Deno.cwd() we get the full path of the OS starting with C:\...

This added a \ before C:\ which resolved in a invalid path :
/C:\Users\xxxx\Documents\Projects\xxxx/assets/manifest.json

Type of change

• Bug fix (non-breaking change which fixes an issue)

---

Checklist:

• I have run deno lint AND deno fmt AND deno task test and got no
  errors.
• I have followed the contributing guidelines of this project as mentioned
  in CONTRIBUTING.md
• I have checked to ensure there aren't other open
  Pull Requests for the same
  update/change?
• I have performed a self-review of my own code
• I have made corresponding changes needed to the documentation

[hironichu]

Wait should this PR be merged in V2 branch.. ?

[Sorikairox]

@hironichu Thank you for your PR !

No worries, the V2 branch is obsolete, main branch is V2.

Unfortunately, your fix indeed fix windows, but also break everything else 😢

I added a condition, if we're not on windows we prepend the /