docs: add VS Code Marketplace badge after publish

[SebTardif]

After the extension is published to the VS Code Marketplace (see #2), add a Marketplace badge to the README:

[![VS Code Marketplace](https://img.shields.io/visual-studio-marketplace/v/patchloom.patchloom?logo=visual-studio-code)](https://marketplace.visualstudio.com/items?itemName=patchloom.patchloom)

Optionally also add an install count badge:

[![Installs](https://img.shields.io/visual-studio-marketplace/i/patchloom.patchloom?logo=visual-studio-code)](https://marketplace.visualstudio.com/items?itemName=patchloom.patchloom)

Blocked on #2 (marketplace publish). Per badge hygiene rule #3, do not add badges that show embarrassing state; add after the extension is live.

Identified by repo-badges skill audit.

[SebTardif]

Marketplace badge has been added to README in PR #73. It currently shows as a fallback shield since the extension is not yet published. The badge will show the real version once the first publish completes.

[SebTardif]

Closed by PR #80. VS Code Marketplace badge was already present in the README.