Implement get_facet_fml() via tinyformula()/tinyframe()

The tinyplot.formula() method uses auxiliary functions tinyformula()/tinyframe() (see https://github.com/grantmcdermott/tinyplot/blob/main/R/tinyformula.R) for processing its formulas, including facets.

In sanitize_facet() (see https://github.com/grantmcdermott/tinyplot/blob/main/R/sanitize_facet.R) an auxiliary function get_facet_fml() (see https://github.com/grantmcdermott/tinyplot/blob/main/R/facet.R#L680-L726) is used for cases when facet is still a formula. However, get_facet_fml() is not implemented using tinyformula()/tinyframe(). It might be a good idea to harmonize/streamline this implementation. This first came up in #562

As a first step, I tried to come up with an example when get_facet_fml() is actually used successfully. Could you provide an example, Grant @grantmcdermott ?

My reading is that this can only happen if tinyplot.default() is used because tinyplot.formula() already parses facet formulas and does not pass these on. But in tinyplot.default() I always get an error if I do tinyplot(x, y, facet = a ~ b) or tinyplot(x, y, facet = a ~ b, data = d).

Compare get_facet_fml() in R/facet.R with tinyformula()/tinyframe() in R/tinyformula.R and its use from R/sanitize_facet.R. Reproduce the facet formula cases through tinyplot.default(), with and without data, to establish a working example. Done means the example works and get_facet_fml() is harmonized with the existing formula-processing helpers.