Add web platform boilerplate [SDK-3983]

[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 adds the web platform interface to the auth0_flutter_platform_interface package, and the respective –empty– implementation to the app-facing auth0_flutter package. For non-web platforms, this implementation is no-op, whereas for the web it allows to call the actual JS interop methods. The existing mobile-related files in the app-facing package were moved under a new mobile directory.

An empty Auth0Web class was also added to auth0_flutter.dart (in the app-facing package).

📎 References

This follows the approach explored in https://github.com/auth0/auth0-flutter/tree/flutter-web/option-2.

🎯 Testing

This was tested manually by adding a small JS library to index.html and calling it from Dart code.

[codecov]

Codecov Report

Base: 98.72% // Head: 98.31% // Decreases project coverage by -0.42% ⚠️

Coverage data is based on head (8a315a2) compared to base (cd5effb).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               beta     #197      +/-   ##
============================================
- Coverage     98.72%   98.31%   -0.42%     
  Complexity       82       82              
============================================
  Files            78       79       +1     
  Lines          1417     1423       +6     
  Branches        313      313              
============================================
  Hits           1399     1399              
- Misses            6       12       +6     
  Partials         12       12              

Flag	Coverage Δ
auth0_flutter	100.00% <ø> (ø)
auth0_flutter_android	96.60% <ø> (ø)
auth0_flutter_ios	100.00% <ø> (ø)
auth0_flutter_platform_interface	97.17% <0.00%> (-2.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
auth0_flutter/lib/auth0_flutter.dart	100.00% <ø> (ø)
...th0_flutter/lib/src/mobile/authentication_api.dart	100.00% <ø> (ø)
...h0_flutter/lib/src/mobile/credentials_manager.dart	100.00% <ø> (ø)
...th0_flutter/lib/src/mobile/web_authentication.dart	100.00% <ø> (ø)
..._interface/lib/src/auth0_flutter_web_platform.dart	0.00% <0.00%> (ø)

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