Add CI to run tests on PRs

[trivikr]

Is your feature request related to a problem? Please describe.
There is not CI enabled in the repo, so a code can get pushed which fails the tests:

• Example PR Convert isa method to an arrow function #163
• Tests fixed in Update Structure Generator Tests #165

Describe the solution you'd like

• Enable CI on smithy-typescript
• Gradle builds can run on TravisCI for example https://guides.gradle.org/executing-gradle-builds-on-travisci/

Describe alternatives you've considered

• Reminding authors to run the tests before posting PR
• Fixing tests in a separate PR, once a PR which breaks tests is pushed

[trivikr]

Closing as we now run CI using GitHub workflow https://github.com/awslabs/smithy-typescript/blob/main/.github/workflows/ci.yml