Improve JetBrains IDE integration #8259

Summary

Improve navigation and project identification in JetBrains IDEs.

Details

Use JetBrains IDEs' built-in project icon and Issue Navigation features:

• .idea/icon.svg displays the RustPython logo as the project icon
• .idea/vcs.xml turns #1234 references in the Git log into links to the corresponding GitHub pull request
• .gitignore tracks only these settings while keeping other .idea files ignored

This makes the project easier to identify and avoids manually searching for referenced pull requests.

References:

• chore: add new icon.svg and vcs.xml and update .gitignore to exclude specific IDEA files apache/fesod#738
• [improve][misc] Add IssueNavigationLink for IDEA git log apache/pulsar#17769

Drafted with AI assistance (GPT 5.6), reviewed by a human before filing, per RustPython's AI policy.