PBRCustomMaterial & CustomMaterial support MaterialPlugin

[Starryi]

PBRCustomMaterial is a subclass of PBRMaterial, but does not support MaterialPlugin.

In the implementation of PBRCustomMaterial, the shader injection point is removed , causing material plugin to be unable to inject custom code.

CustomMaterial has the same problem.

This playground https://www.babylonjs-playground.com/#VNLCRN can reproduce the issue.
In the playground, PBRCustomMaterial using MeshDebugPluginMaterial, does not rendering wireframe.

The playgroud rendering result is as follows

red mesh use PBRCustomMaterial, green mesh use PBRMaterial
yellow mesh use CustomMaterial, blue mesh use StandardMaterial

I hope to fix the problem by this commit.
In the commit,

1. reorganize the shader creation in PBRCustomMaterial and CustomMaterial
2. replace the shader code only if the corresponding injected code is not empty
3. retain injection points

After fixing this issue, the above plaground rendering result is as follows

[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]

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[Popov72]

Only one comment and we are good to go!