fix: add type for function localsConvention value

[andrewiggins]

Description

PostCSS Modules supports a function as a value to localsConvention. This PR adds typing for that function. See the postcss modules documentation here: https://github.com/madyankin/postcss-modules#localsconvention

Additional context

Nope

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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