af: Add symlinks to `ios` and `macos` [SDK-4693]

[Widcket]

• [ ] All new/changed/fixed functionality is covered by tests (or N/A)
• [ ] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR symlinks all the files within the darwin directory into the ios and macos directories. This is necessary because pub does not support symlinking directories, so each file has to be symlinked individually.

The .gitignore file inside darwin was copied instead of symlinked, because of Git behavior:

Commits:

• https://github.com/auth0/auth0-flutter/commit/413943038dee18e1e1f26ff6d9b0bc40ad6386bd adds the symlinks to the ios directory
• https://github.com/auth0/auth0-flutter/commit/a2091c237306aaa549a068d60b2dc1ed2e1bd78f adds the symlinks to the macos directory

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:exclamation: No coverage uploaded for pull request base (feature/macos-support@b2370e5). Click here to learn what that means.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/macos-support #350 +/- ## ======================================================== Coverage ? 96.36% ======================================================== Files ? 98 Lines ? 1596 Branches ? 328 ======================================================== Hits ? 1538 Misses ? 50 Partials ? 8 ``` | [Flag](https://app.codecov.io/gh/auth0/auth0-flutter/pull/350/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [auth0_flutter](https://app.codecov.io/gh/auth0/auth0-flutter/pull/350/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `100.00% <0.00%> (?)` | | | [auth0_flutter_android](https://app.codecov.io/gh/auth0/auth0-flutter/pull/350/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `97.35% <0.00%> (?)` | | | [auth0_flutter_ios](https://app.codecov.io/gh/auth0/auth0-flutter/pull/350/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `99.85% <0.00%> (?)` | | | [auth0_flutter_platform_interface](https://app.codecov.io/gh/auth0/auth0-flutter/pull/350/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `87.03% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#carryforward-flags-in-the-pull-request-comment) to find out more.

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