Feature: support of Javascript fixtures

[sraone]

Description

Modified below lines to support .js fixture files.

https://github.com/marko-js/fixture-snapshots/blob/master/src/index.ts#L34
https://github.com/marko-js/fixture-snapshots/blob/master/src/index.ts#L42

Motivation and Context

#1

Screenshots (if appropriate):

Checklist:

• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[jsf-clabot]

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