Reduce dependencies

[jeroenjanssens]

Would be good to reduce the number of dependencies. For instance:

• [x] rlang (only rlang::abort() is used; perhaps make it optional or replace with base::stop()?)
• [x] glue (glue::glue() can perhaps be replaced with paste0()?)
• [x] dplyr (only dplyr::case_when is used; perhaps replace with good old if...else?)

Moreover, some demos currently depend on packages that are not listed as a dependency in DESCRIPTION:

• [x] purrr
• [x] fs
• [x] stringr

We should either add them as dependencies or replace the code with a base alternative.

[jeroenjanssens]

Removed {fs}, {glue}, {stringr}, {dplyr}, and {purrr} as dependencies from the demos (see 6e3458ab14fa8a1dab0b11324b2de4a923e7750b).

[jeroenjanssens]

Removed {glue} and {dplyr} as dependencies (see 70bec409355d8078c52f2ddefb2e30889d599ec6 and b13a69d358dc8f5044ce550861500c885bbe8afe).