[WIP] Revised version of R 'transpiler' with support for generator methods

[rpkyle]

OK, I've taken a stab at using the new generator method approach to component generation for R. If you two don't mind having another look, would be much appreciated. Closes #445.

@rmarren1 @T4rk1n

(N.B. This PR is the up-to-date version of this closed PR)

[T4rk1n]

• Move big templates string as global at top of the file to make methods logic more readable.
• Improve the docstrings.
• Add --r-prefix option.
• Call os.makedirs only once in component_generator.
• Imports at top of files.
• Copy the whole js/css dist.
• generate_rpkg should use .get for field access, can't be sure that a package.json will contain some the fields used.

[T4rk1n]

Looking good, just need to fix json_to_r_type and I think we're good to go.

[rmarren1]

Few comments, looks good overall!

[rpkyle]

Thanks for the additional reviews, I think the code has improved a lot -- both due to the useful factoring of the component generation in dash, but also because of the suggestions here and in the earlier PR.

@T4rk1n and I had a nice chat about json_to_r_type, before we 💃, I think I need to make some changes to this method, to handle two cases I missed previously:

• if ['type']['name'] is custom, and the value is raw, the parameter type in R should be numeric() (or, more specifically, could be integer() or numeric())
• if ['type']['name'] is enum, a bit more complicated to address: need to check option type in the value field, i.e. if option is 'click' the option type should be character(), but it could also be numeric(), or even an object in JSON

As for the other options:

• if ['type']['name'] is func, node, or object, we're passing NULL for the time being

[rpkyle]

@T4rk1n Following up on our conversation from Friday, how does this version of props_to_r_type look?

[rpkyle]

@T4rk1n Following up on our conversation from Friday, how does this version of props_to_r_type look?

Following productive discussion with @T4rk1n, PR appears ready to merge. Will now do so as all requested changes to code have been made.