Add XDG base directory spec support

[Alrefai]

• fix(test): add NPM_CONFIG_CACHE dir support

The 'should install an npm module transparently' test did not account for the possibility that the NPM_CONFIG_CACHE environment variable might be set. So, it would fail in that case. The fix includes checking the value of NPM_CONFIG_CACHE and using that value if it is set, otherwise defaulting to .npm directory.

Summary of all failing tests
 FAIL  packages/mrm/src/__tests__/index.spec.js (12.65s)
  ● resolveUsingNpx › should install an npm module transparently

    : Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Error:

      102 | 
      103 | describe('resolveUsingNpx', () => {
    > 104 |     it('should install an npm module transparently', async () => {
          |     ^
      105 |             const result = await resolveUsingNpx('yarnhook');
      106 |             expect(result).toMatch(
      107 |                     /\.npm\/_npx\/\d*\/lib(64)?\/node_modules\/yarnhook\/index\.js$/

      at new Spec (node_modules/jest-jasmine2/build/jasmine/Spec.js:116:22)
      at Suite.<anonymous> (packages/mrm/src/__tests__/index.spec.js:104:2)
      at Object.<anonymous> (packages/mrm/src/__tests__/index.spec.js:103:1)
      at processTicksAndRejections (node:internal/process/task_queues:95:5)

  ● resolveUsingNpx › should throw if npm module is not found on the registry

    expect(received).toMatch(expected)

    Expected pattern: /\.npm\/_npx\/\d*\/lib(64)?\/node_modules\/yarnhook\/index\.js$/
    Received string:  "/Users/mohammed/.cache/npm/_npx/23946/lib/node_modules/yarnhook/index.js"

      104 |     it('should install an npm module transparently', async () => {
      105 |             const result = await resolveUsingNpx('yarnhook');
    > 106 |             expect(result).toMatch(
          |                            ^
      107 |                     /\.npm\/_npx\/\d*\/lib(64)?\/node_modules\/yarnhook\/index\.js$/
      108 |             );
      109 |     });

      at Object.<anonymous> (packages/mrm/src/__tests__/index.spec.js:106:18)
      at processTicksAndRejections (node:internal/process/task_queues:95:5)

• feat(config): add XDG base directory spec support

This change enables users to organize their config file according to the XDG Base Directory specification.

• docs(md): add XDG base directory instructions