[Feature Request] Allow unnamed snippets

[lorensr]

Is your feature request related to a problem? Please describe.

Unable to use snipsync in files that don't support // comments: temporalio/samples-typescript#212 (comment)

Describe the solution you'd like

<!--SNIPSTART <github URL> <highlight and/or lines selection> -->
<!--SNIPEND-->

e.g.:

<!--SNIPSTART https://github.com/temporalio/samples-typescript/blob/main/hello-world/package.json -->
<!--SNIPEND-->