fix: allow parsing of empty keys in objects

[Coobaha]

Resolves #432

• [x] to add an option to both parse and stringify
• [ ] to have tests, for both parse and stringify, for at least:
  • [x] repeat keys with all array formats
  • [x] bracketed keys with all array formats
  • [x] a lone = (and =& and &= and &=&)
  • [ ] ❌ deep array/object case

If we are happy with this implementation and want to proceed:

• [x] to document in the readme
• [x] split tests before merge to parse/stringify files

• [ ] update https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/qs (also allowSparse is missing there and maybe other types)

  Other:

• Stringify works without an option, so it is potentially a patch but altering parse behaviour only via option seems a safe bet to not accidentally break ppl code, and can be considered as a default behaviour in the next major version?

[ljharb]

There's unlikely to be a major version any time soon; breaking changes just aren't worth it. Configuring it via option is the best bet for now.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a5609c7) 99.80% compared to head (5e70a6a) 99.80%.

:exclamation: Current head 5e70a6a differs from pull request most recent head c077991. Consider uploading reports for the commit c077991 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #433 +/- ## ======================================= Coverage 99.80% 99.80% ======================================= Files 9 9 Lines 1518 1531 +13 Branches 177 179 +2 ======================================= + Hits 1515 1528 +13 Misses 3 3 ``` | [Impacted Files](https://app.codecov.io/gh/ljharb/qs/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jordan+Harband) | Coverage Δ | | |---|---|---| | [test/empty-keys-cases.js](https://app.codecov.io/gh/ljharb/qs/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jordan+Harband#diff-dGVzdC9lbXB0eS1rZXlzLWNhc2VzLmpz) | `100.00% <ø> (ø)` | | | [lib/parse.js](https://app.codecov.io/gh/ljharb/qs/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jordan+Harband#diff-bGliL3BhcnNlLmpz) | `100.00% <100.00%> (ø)` | | | [test/parse.js](https://app.codecov.io/gh/ljharb/qs/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jordan+Harband#diff-dGVzdC9wYXJzZS5qcw==) | `99.80% <100.00%> (+<0.01%)` | :arrow_up: | | [test/stringify.js](https://app.codecov.io/gh/ljharb/qs/pull/433?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jordan+Harband#diff-dGVzdC9zdHJpbmdpZnkuanM=) | `99.79% <100.00%> (+<0.01%)` | :arrow_up: |

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