[`avar`] Write proposal for enhancement and legalizing currently illegal cases

[behdad]

Too enable:

• Moving min/default/max position of a font via avar, by removing the requirement to map -1:-1, 0:0, +1:+1.
• Enable abrupt jumps, by allowing same value to map to up to three values.
• Enable extrapolation, by allowing values to map to full [-2,+2) range, instead of current [-1,+1].
• While at it, remove the requirement that mapped-to values be non-decreasing.

[behdad]

cc @Lorp @letterror

[behdad]

cc @twardoch @justvanrossum

[behdad]

See extensive discussion in https://github.com/harfbuzz/boring-expansion-spec/issues/89

cc @nedley @PeterCon