Linearize support version info

[saschanaz]

This is related to #1596.

Currently the version info becomes "parallel" when it comes to the legacy webkit aliases.

In flex-basis:

This is correct but the reader needs to get the information in "parallel" way:

• 22+ just supports it
• 49 added -webkit- alias
• 44 added -webkit- alias behind flag
• 18 supported behind flag and removed the flag in 28 (while the above confusingly does not say when the flag was removed)

This is needlessly complex, I think the version info should be linear per item:

• 49 supports both the original property and the alias (could be helpful if alternative_name can indicate whether the original name also exists)
• 44 added alias behind flag
• 22 added the support
• 18 supported the support behind flag (and forget when the flag was removed. Use version_removed only when the feature itself is removed)

Or even split them (but having two sources for prefixes (alternative_name or a subitem) is not very good):

• flex-basis:
  • 22 added the support
  • 18-21 supported the support behind flag
• -webkit-flex-basis:
  • 49 supports both the original property and the alias (could be helpful if alternative_name can indicate whether the original name also exists)
  • 44-48 added alias behind flag

Changes in #7401 mainly has this kind of complexity. What do you think?

[ddbeck]

This feels like one of those things that would be less clumsy if we could tombstone dead flags. See #3318.

In this case, if we could mark the entry about 44 adding the -webkit- alias behind flag as removed in version 49, the version history would be a lot cleaner, with only two open-ended support statements: one prefixed and one unprefixed.