STRF-8747: split .stencil file into 2 separate config files

[MaxGenash]

What?

• Moved code dealing with .stencil file into a separate class.
• Split .stencil file into 2 separate config files: secrets.stencil.json (contains "accessToken", "githubToken") and config.stencil.json (all other fields), so that themes developers can optionally keep general config data (config.stencil.json) in git and add secrets.stencil.json to .gitignore. Also implemented migration from an old config file format on config read to avoid breaking changes.
• Increased coverage threshold since we have a higher level at the moment.

Tickets / Documentation

Ticket: STRF-8747
Issue: #523

[bookernath]

👏 thanks for doing this.

As some existing users of CLI may be expecting the .stencil file to be used, we should probably release this as a new major version, 4.0.

[bookernath]

@MaxGenash can you also add these files to .gitignore in Cornerstone?

https://github.com/bigcommerce/cornerstone/blob/master/.gitignore#L2

This will help make sure theme developers do not accidentally commit them to source control.

[MaxGenash]

@bookernath I implemented smooth backward compatibility, so at the moment when users reference the .stencil file in any CLI command they will receive a message about updating the file, it will be automatically replaced and the users can continue working with CLI, no breaking changes yet.
So I believe we can release these changes as v3.1.0 (feature update) and later drop .stencil file support in the v4 release

[bookernath]

@MaxGenash great, good thinking and thank you for clarifying!

[junedkazi]

@MaxGenash can you make sure we are not pushing the credentials file when we do a stencil push or when we make a bundle

[MaxGenash]

@junedkazi checked, we are not pushing the credentials file when we do a stencil push or when we make a bundle

[MaxGenash]

@bookernath @junedkazi before releasing these changes, need to update the documentation on developer.bigcommerce.com to replace mentions of .stencil with the new files and update videos on YouTube where .stncil file is edited (e.g. https://youtu.be/qgaDX7bhmd8 )