feat(python,rust): include set_fmt_float value in Config load/save state

[alexander-beedie]

Allow current state of the associated Config.set_fmt_float value to be saved; this also allows it to work properly in context-manager scope.

(Also, declares a TypeAlias for FloatFmt and sorts some of the Config methods).

Example:

with pl.Config() as cfg:
    cfg.set_fmt_float("full")
    print(df)

# float fmt reset to "mixed" (original state) on scope-exit