Web component: Add parts

[strarsis]

Check that this is really a bug

• I confirm

Reproduction link

https://codepen.io/strarsis/pen/qBJNJzv

Bug description

The Swiper Element (Web Component) does not have parts for its sub-components, as its pagination (pagination button (normal / active)), navigation, etc.
This makes styling those elements unnecessarily hard, as selecting those elements in shadow DOM by CSS is not directly possible from the outside.

Expected Behavior

All sub-components should be parts, so that styling is very easy and efficient,
it also leverages the strengths of such web components.

Actual Behavior

No parts are available.

Swiper version

9.2.3

Platform/Target and Browser Versions

Chrome (latest stable at 18.04.2023) on Windows 10 x64

Validations

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.
• Make sure this is a Swiper issue and not a framework-specific issue

Would you like to open a PR for this bug?

• I'm willing to open a PR

[nolimits4web]

Customization must be done through CSS properties, if there are not enough CSS properties, you can open another feature request suggesting what to add.

[nolimits4web]

Added