Fixed Playground loading on custom SnippetID

[Tricotou]

In my last PR adding the procedural code generator, I inserted the JSON fetch in the monacoEditor constructor, to make sure it was loaded first, but the _load function was called after the fetch, which could lead to "forever" engine loading UI in the event of loading a custom PG.

• This PR fixes the previous issue. (--> _load before fetch)
• It also fixes the potential cache issue by adding an uncacher param to the JSON urls (since it's very light) :
  const url = "templates.json?uncacher="+Date.now();
• Missing "\n" at beginning of generated code was added

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/15264/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15264/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15264/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/15264/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/15264/merge
https://gui.babylonjs.com/?snapshot=refs/pull/15264/merge
https://nme.babylonjs.com/?snapshot=refs/pull/15264/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/15264/merge#BCU1XR#0

[bjsplat]

WebGL2 visualization test reporter:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15264/merge/testResults/webgl2playwright/index.html

[bjsplat]

Visualization tests for WebGPU (Experimental)

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15264/merge/testResults/webgpuplaywright/index.html

[Tricotou]

@RaananW for your information, if you need to access a custom playground but run into the loading bug, you can access a clean playground (https://playground.babylonjs.com) and then add the ID to it