should the headers include an extension version?

Since we have started to include the extension version in the extension specs, should we include the extension version in the headers also? This will help applications to know what is the maximum version of the extension their headers support, vs. checking for the presence or absence of features that may be available only in newer versions of the extension individually.

This is done by some Khronos specs already. For example, the headers for a Vulkan extension include a line similar to:

```c
#define VK_KHR_SURFACE_SPEC_VERSION 25
```

If we decide to do this, ideally we would include the version in the XML file and generate it in the headers (#113, #161).

Start by reviewing issues #113 and #161, then inspect how the XML specification relates to generated headers. Determine whether extension versions should be represented in the XML and exposed as header macros, and document the agreed scope and generation behavior as the completed outcome.

Written by the indexing model from the issue text.