Replace ggh4x with legendry

[teunbrand]

Hi Grant,

I'm the author of ggh4x and I am sunsetting the ggh4x::guide_*() family of functions.
Because ggfixest uses ggh4x::guide_axis_nested(), this may cause problems for ggfixest.
This PR replaces ggh4x's guides with legendry's guides, which is the successor of ggh4x's guides.
Unfortunately there is not a 1:1 translation from ggfixest's use case to an easy legendry guide, so it takes a little bit more wrangling to get the guide to display correctly.

Best wishes,
Teun

[grantmcdermott]

Thanks @teunbrand. legendary looks cool and I very much appreciate the PR.

Two requests (one minor, one less so):

1. Can we also remove the line width call here https://github.com/teunbrand/ggfixest/blob/07dddc1536f66f7c18f77a6efaf856ac08f17c05/R/ggiplot.R#L479-L482? I don't think legendary has an equivalent setting but it probably doesn't matter much.
2. Right now, all the tests are failing. Ideally we could regenerate them as part of this PR. This is pretty simple thanks to tinysnapshot: just delete the inst/tinytest/_tinysnapshot folder and then run tinytest::test_all() to regenerate... but it does need to be done Linux (a Docker container is fine). Are you able to do this?

[teunbrand]

Sure; 1) is a good call and 2) I did it on WSL (I can't be bothered to deal with docker at the moment)

[grantmcdermott]

Thanks @teunbrand!