Whats new screen added

[ric-v]

This pull request updates the extension version and improves the robustness of the changelog loading logic. The most important changes are summarized below:

Version update:

• Bumped the extension version in package.json from 0.7.6 to 0.7.9.

Changelog handling improvements:

• Enhanced the WhatsNewManager to attempt loading the changelog from multiple filename variants (CHANGELOG.md, changelog.md, Changelog.md), improving compatibility with different file naming conventions. If none are found, it now lists the files in the extension root for easier debugging.

.vscodeignore adjustments:

• Modified .vscodeignore to remove CHANGELOG.md from the ignore list and add MARKETPLACE.md, potentially ensuring the changelog is included in the packaged extension.