build(load-files): generic for loadFilesSync and loadFiles

[jjangga0214]

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Description

• loadFilesSync has generic while loadFiles. So generic is added to loadFiles.
• Instead of <T = any>, <T = DocumentNode> is used, since they are mainly used for loading schema. This even makes it work with graphql-module.

import { createModule } from 'graphql-modules'

export default createModule({
  // `typeDefs` requires `DocumentNode[]`. 
  // Before, `loadFiles` caused type error as it's `any[]`. 
  // But by this PR, it works as expected.
  typeDefs: await loadFiles('...'),
})

Type of change

Please delete options that are not relevant.

• 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 not work as
  expected)
• This change requires a documentation update

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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