Introduce agreed code style

[matt-dray]

Code style can be arbitrary, but it might be beneficial to set a few early 'ground rules' that could prevent confusion and breaking changes later on, while making the package easier to develop and use.

A small example: it may be preferable to start function names with a verb, since that will make clear to users exactly what the function is doing.

Perhaps the tidyverse style guide? {styler} and {lintr} packages could be used to automate consistency, including via GitHub Actions (see #8).

More broadly, if CRAN is the aim, then we'll need to adhere to the Writing R Extensions guidelines, which requires specifics like title case for the 'Title' in the DESCRIPTION file.