Upgrade bitstyles to 6.0.0

[angelikatyborska]

Based on top of #125

Resolves #110 #108 #109

[angelikatyborska]

@andreasknoepfle I ran into a surprise: the class replacements from the classnames helper do not chain... For example, there was this chain of renaming:

u-fg-grayscale-dark-2 (6.0.0) -> u-fg-text-darker (5.0.0) -> u-fg-gray-50 (4.3.0)

If I render a component that uses the class u-fg-grayscale-dark-2, and I render it for v4.3.0, it ends up with the class u-fg-text-darker instead of u-fg-gray-50 because replacements are applied backwards 🤔 (first from 5.0.0 -> 4.3.0, then from 6.0.0 -> 5.0.0).

[angelikatyborska]

I ran into a surprise: the class replacements from the classnames helper do not chain... For example, there was this chain of renaming:

Will be resolved by merging #134 first

[angelikatyborska]

I'm pretty confident of those changes because we've been using them in the IDA backend with no hiccups

[andreasknoepfle]

Changes look awesome. The only thing missing for me is an entry to the CHANGELOG.md