feat(stylelint-config): add @modular-css/stylelint-config package

[tivac]

Description

Adding a new package, @modular-css/stylelint-config to make it easy to have stylelint ignore the modular-css custom syntax using its extends support.

Motivation and Context

I don't want to keep on having to do this manually every time I start a new project using stylelint & modular-css.

How Has This Been Tested?

Been using these properties as part of our broader stylelint config on a project for over a year. Extracted from that and then tested within that project.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #637 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #637   +/-   ##
=======================================
  Coverage   99.13%   99.13%           
=======================================
  Files          46       46           
  Lines        1150     1150           
  Branches      174      174           
=======================================
  Hits         1140     1140           
  Misses         10       10

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