UserFlags should include 0 as a member

[ghaaj]

Issue description

UserFlags can be 0 when the user has no flags, but there is no such member in the current definition.

Code sample

Package version

0.37.120

Runtime

Other / Browser

Runtime version

workerd@1.20250408.0

Priority this issue should have

Low (slightly annoying)

[Qjuh]

The enum shouldn’t include that though. It‘s a valid value for all properties that have UserFlags as type though. Unless you provide an actual case where this is an issue