Improve generation of function template variable names

[zth]

We're able to generate function template/snippets for callbacks since a while back. But, given that the type information has no name for unlabelled arguments, we've been forced to call most variables in the generated template v (with a few notable exceptions that we've special cased).

This PR improves how we infer template variable names by looking at the type path. For example, the type Environment.t would be called environment in templates, whereas Environment.envConfig would be called envConfig.

This can help the DX quite a lot, because we can capture some of the variable name intent, at least for more complex types.

Before:

After:

I think this will be quite useful, but interested in hearing more thoughts.

[cristianoc]

Looks like CI is red.

[zth]

No idea why CI is red now, everything passes locally. Will try and figure out tomorrow.

[zth]

@cristianoc I removed the custom attribute stuff, and just went with improving the template variable names for now.