Add a way to create an immutable snapshot of a BitSet

It is possible to provide an immutable BitSet backed by a `@Stable` array, enabling constant-folding optimizations.

The following classes in the JDK would potentially benefit from such an addition:

ShapedTranslucentWindowClick (1)
PoolReader (6)
URLEncoder (1)
JMXServiceURL (5)
TableView (1)
HtmlTree (2)

Parentheses indicate the number of `static final` usages where an immutable BitSet might be used.