typekits `is` APIs should accept Entity or unknown

[chrisradek]

Currently - many of the typekit is APIs are too restrictive in what they accept. For example, $.value.is realistically only accepts a Value type:

typespec/packages/compiler/src/experimental/typekit/kits/value.ts

Line 100 in 77b1b5a

is(type: { valueKind: string }): type is Value;

This makes it really hard to use these APIs with other methods that return Type | Value or Entity to narrow the types.

I think in general we should encourage that is* APIs support passing in either Entity or unknown.

[markcowl]

est: 3