Add clip spinner type to dcc.Loading

[budescode]

Adds clip as a new built in spinner type for dcc.Loading: a ring with a gap that rotates and pulses slightly. Use it with dcc.Loading(type="clip").

dcc.Loading ships 5 built-in types, and none of them is a plain rotating ring, which is the most common loading indicator on the web.

The spinner is a port of ClipLoader from react-spinners (MIT). It follows the same approach as the existing default, dot and circle spinners, which are ports from SpinKit: no new dependency, one self-contained file with inline CSS, and the source credited in a comment.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Add ClipSpinner.tsx
  • Register clip in Loading.tsx and in the type union in types.ts
  • Add integration test test_ldcp020_loading_component_clip_spinner
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
17.5% Duplication on New Code

See analysis details on SonarQube Cloud