luci-base: form.js: add support for dynamic default values

Many uci options have different implicit defaults, depending on the
values of other related options. A prominent example is the wireless
ieee80211w option which defaults to 2, 1 or 0 depending on whether
the encryption is set to wpa3-sae, wpa2/wpa2 mixed or any other mode
respectively.

Add the ability to specify a dictionary of default values and their
corresponding dependencies in order to be able to express such
configurations.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>