[feaLib] Support 'u', 'd', and 'n' suffixes for specifying axis locations in user/design/normalized space

[anthrotype]

currently conditionset blocks for specifying FeatureVariations use user-space axis coordinates in the Variable FEA spec proposal, and that's how feaLib implmented them: adobe-type-tools/afdko#1350

However it's useful to be able to specify them in either internal design-space or normalized space, and @skef proposed the following optional suffixes 'u', 'd', and 'n' to specify whether axis locatations should be interpreted respectively in user/design/normalized space: harfbuzz/boring-expansion-spec#94 (comment)

fea-rs has already implemented this, see cmyr/fea-rs#203

feaLib should add support for these suffixes as well

[behdad]

Good luck compiling u with avar2! :(