Load custom snippet syntax by relative path only if it begins with a dot

[charlierudolph]

Want to implement something similar to #1413 for snippet syntax.

---

The above-mentioned change made referencing custom formatters a lot more logical. Before:

# an npm package
--format node_modules/some-formatter-package/index.js

# a project-local file
--format formatter.js

After:

# an npm package
--format some-formatter-package

# a project-local file
--format ./formatter.js

We want it to work the same way for custom snippet syntaxes, but it's technically a breaking change so should probably wait for 8.0.0.

[aurelien-reeves]

@charlierudolph may I let you qualify that issue using labels?

Also if you could add some context and/or description, that would be great 😄

[davidjgoss]

@aurelien-reeves updated!