Change density default `bw = "nrd0"` to `bw = "sj"`

[teunbrand]

This PR aims to fix #3825.

Briefly, ?density recommends the "sj" method over the "nrd0" default. This change propagates that recommendation to ggplot2's density calculations.

Some details:

• Understandably, plenty of snapshots have changed.
• I realised that calc_bw() and precompute_bw() were doing the exact same task, so I merged these.
• When var(x) == 0, most of the stats::bw.*() functions throw an error, so we keep bw.nrd0() for these degenerate cases because it will not throw an error.