[ci] place all CI helpers under the .ci folder and use - instead of _ in their names

[StrikerRUS]

Simplify repo's structure in the following way:

1. Move all CI scripts under .ci folder
2. use - instead of _ in scripts' names to ensure their correct ordering

breaking tag due to "public" build-r.R script.

[borchero]

What's the benefit of this change? 😄

[StrikerRUS]

@borchero Simplification of the repo's structure. All helper scripts used in our CI are placed in one directory. Newcomers won't struggle deducing the difference between helpers and .ci folders, for example.

[borchero]

@StrikerRUS ok, I only briefly looked at the file changes before and it seemed like it would only include renames from snake case to kebab case. I see that it actually moves three files 😄

I guess (1) moving files makes a lot of sense, I'm not too sure about (2) changing the file names but I don't really mind.

[jameslamb]

I support the idea of removing helpers/ and just consolidating everything into ci/. That will definitely reduce a bit of friction.

I also support preferring - to _, as it requires one less keypress (it's a minor thing, but still nice!).

However.... I think we should keep build_r.R using snake_case. As you noted, it sort of is "public". It's the documented, preferred entry point for building custom variations of the R package (like with GPU support). I don't support breaking peoples' builds in exchange for a bit more consistency in the naming of scripts.

[StrikerRUS]

@jameslamb

I think we should keep build_r.R using snake_case.

Reverted in d0dbc63.

[jameslamb]

great, thank you