Switch to export default syntax for exporting rules

[bmish]

For module files (which import statements instead of require), we might as well use export statements instead of module.exports to adopt fully modern syntax.

As suggested in: #818 (comment)

[codecov]

Codecov Report

Merging #819 (146b6be) into master (d4a57d8) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 146b6be differs from pull request most recent head df34872. Consider uploading reports for the commit df34872 to get more accurate results

@@ Coverage Diff @@ ## master #819 +/- ## ========================================== - Coverage 99.24% 99.22% -0.02%  ========================================== Files 98 98 Lines 1453 1417 -36 Branches 477 477 ========================================== - Hits 1442 1406 -36  Misses 11 11 

Impacted Files	Coverage Δ
src/rules/accessible-emoji.js	100.00% <ø> (ø)
src/rules/alt-text.js	100.00% <ø> (ø)
src/rules/anchor-has-content.js	100.00% <ø> (ø)
src/rules/anchor-is-valid.js	100.00% <ø> (ø)
src/rules/aria-activedescendant-has-tabindex.js	100.00% <ø> (ø)
src/rules/aria-props.js	100.00% <ø> (ø)
src/rules/aria-proptypes.js	93.22% <ø> (-0.12%)	⬇️
src/rules/aria-role.js	100.00% <ø> (ø)
src/rules/aria-unsupported-elements.js	100.00% <ø> (ø)
src/rules/autocomplete-valid.js	100.00% <ø> (ø)
... and 26 more

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[bmish]

Not so sure how this change could reduce code coverage. Maybe a flake?

[ljharb]

@bmish turns out this caused #824 - iow, it caused the built files to export { default: rule } instead of rule directly.

[bmish]

@ljharb thanks for fixing it!