add a workflow to check clang-format

[bkietz]

Adds a convenience make format target to call clang-format on **/*.hpp. Checks for clang-format-10 specifically to avoid the subtle differences between clang-format versions.

Concerns:

• I'd like to run this on **/*.cpp as well, but cpp11test/src/cpp11.cpp is generated and I'm not sure formatting it is appropriate. Also cpp11test/R/cpp11-bindings.cpp would match that glob and is also generated but it seems it should be named cpp11test/R/cpp11-bindings.R? Generated code is marked with // clang-format off
• I've used make conditionals and find to implement globbing, version checking, etc. It might be preferable to move that logic to an R script
• We'd probably like to add the same functionality for the R source using formatR or similar

[jimhester]

Should this use run-clang-format.py? (https://github.com/Sarcasm/run-clang-format#how-to-use) it supports a .clang-format-ignore file to exclude files.

Alternatively (and maybe a better suggestion) is maybe we should just add a // clang-format off comment in the generated code

[jimhester]

I think I will add //clang format off to the generated code, it is the simplest solution.

EDIT done in 0e0442d

[jimhester]

Looks good, even picked up an issue :)

I will merge this and fix up the formatting issue after the merge

Thanks!