add windows core package detection

[Aerijo]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

On Windows the snippet file paths use \ as the delimiter. This doesn't match the /node_modules/ regex, so community snippets don't get priority.

The change sets the regex depending on if the OS is Windows or not.

Root cause

• Last known to pass on 1.46.0-beta0 and 1.45.0 (Mar 11 2020, #305), but break on 1.48.0 (Jun 10 2020, #306).
• The package paths come from the package-manager module in Atom, which creates the list using an UnderscoreJS object keys to array method, which is ultimately a call to Object.keys.
• Notable changes: Electron 4.2.7 -> 5.0.12
• Corresponds to Node 10.11.0 -> 12.0.0

I don't have the patience to work out exactly what caused the issue, but a jump in Node means something could have happened to the Object.keys behaviour, the order of which is unspecified for those versions.

Alternate Designs

None

Benefits

Priority test passes

Possible Drawbacks

I don't know why this issue suddenly appeared. It came up in #306, but the cause does not seem to be related to that PR. The check itself has existed (in similar form) for years. The path comes from the Atom package manager module, so perhaps it used to normalise them? Or the files just happened to be in the right order other times?

Applicable Issues

None

[Aerijo]

The docs have isBundledPackage as a method on the package manager. It may be better to replace the regex test entirely, and use the builtin method instead. I just don't know if using it has already been considered at some point and found to be lacking.