Added emscripten support for the Nim's gen script

[Nazariglez]

Related to floooh/sokol-nim#31

[floooh]

Thanks for the PRs! I'm a bit busy atm with this Chrome WebGL2 regressions thingy (there goes the weekend lol), but I'll try to focus on those PRs some time next week.

[floooh]

NOTE: the CI tests are expected to break because they don't have the changes in sokol-nim, don't worry about that.

[Nazariglez]

There is not rush at all, thanks @floooh!

[floooh]

...checking now whether the CI runs through with the changes in sokol-nim merged, this will take a little bit...

[floooh]

...ok actually got a real error in the CI pipeline now:

sokol/gfx.nim(2032, 5) Error: expected: ':', but got: 'keyword when'

...I'll have a look locally...

[floooh]

There's this difference between the generated version and sokol-nim:

diff --git a/src/sokol/gfx.nim b/src/sokol/gfx.nim
index bc8f184..b4758a0 100644
--- a/src/sokol/gfx.nim
+++ b/src/sokol/gfx.nim
@@ -2006,7 +2006,7 @@ when defined emscripten:
   const d3d11* = false
   const metal* = false
   const emscripten* = true
-elif defined gl:
+elif when defined gl:
   const gl*    = true
   const d3d11* = false
   const metal* = false

I guess that when shouldn't be there, I'll try to fix that locally.

[floooh]

Ok, I did this minor fix and then merged: b073c61

I didn't actually try to build for Emscripten, only for native macOS.

I'll add a little blurb to the changelog next.

Thanks for the PR!