Fallback behavior of KHR_techniques_webgl

I want to define both `KHR_techniques_webgl` and `KHR_materials_unlit` and expect that:
1. If the client supports `KHR_techniques_webgl`, use the custom shaders.
2. Otherwise, fallback to `KHR_materials_unlit`

But currently it will not work as expected because the `KHR_materials_unlit` spec says:
> **Implementation Note:** When `KHR_materials_unlit` is included with another
> extension specifying a shading model on the same material, the result is
> undefined.

Is it possible to define (or add an implementation note about) the fallback behavior of `KHR_techniques_webgl`?

Start by reading the KHR_techniques_webgl and KHR_materials_unlit specifications cited in the issue, then review the three existing comments for any agreed direction. Define whether a supported custom shading model takes precedence and how fallback should be specified; done means the specifications contain an unambiguous, accepted rule.

Written by the indexing model from the issue text.