Add eslint and fix linter issues

[susnux]

• Add eslint + config
  • Add linting scripts
  • Fix all eslint issues
  • Add lint CI workflow

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (407abfd) 100.00% compared to head (9498817) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #631 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 40 40 Branches 5 5 ========================================= Hits 40 40 ``` | [Impacted Files](https://app.codecov.io/gh/nextcloud/nextcloud-axios/pull/631?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [lib/index.ts](https://app.codecov.io/gh/nextcloud/nextcloud-axios/pull/631?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#diff-bGliL2luZGV4LnRz) | `100.00% <100.00%> (ø)` | | | [lib/interceptors/maintenance-mode.ts](https://app.codecov.io/gh/nextcloud/nextcloud-axios/pull/631?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#diff-bGliL2ludGVyY2VwdG9ycy9tYWludGVuYW5jZS1tb2RlLnRz) | `100.00% <100.00%> (ø)` | |

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