feat(smithy-client): add isInstance method for ServiceException class

[siddsriv]

Issue #, if available:
Internal JS-5478

Description of changes:
Adds an isInstance type check helper method for generic ServiceException class in smithy-client.

ServiceException.isInstance(e)/ e instanceof ServiceException should now tell if something is an instance of the generic ServiceException class

If one or more of the packages in the /packages directory has been modified, be sure yarn changeset add has been run and its output has
been committed and included in this pull request. See CONTRIBUTING.md.

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