Existing test coverage lacked tests that exercised _both_
the default `processPropertyIdentity` strategy alongside the optional
`propertyIdentityOrBooleanAttribute` strategy.

This commit doesn't contain any implementation changes, but broadens
test coverage instead.

Add an entry to the `src/index.ts` to export the new
`InnerTemplatePart`.

Since consumers are most likely to import `TemplateInstance`, change the
`test/template-instance.ts` test file to import properties from the
module in a way that more closely resembles the end-user consumer
experience.

Export `InnerTemplatePart`

Add test coverage for both built-in processor strategies