fix: migrate playwright-go to mxschmitt module path after deprecation

[faisalaffan]

Problem

github.com/playwright-community/playwright-go is deprecated. The project
has moved to github.com/mxschmitt/playwright-go.

Additionally, Microsoft CDN has removed all Playwright driver binaries
for versions 1.47 through 1.60, causing d2 init-playwright to fail with:

err: failed to install Playwright: could not install driver: could not install driver:
error: got non 200 status code: 404 (404 Not Found) from
https://playwright.azureedge.net/builds/driver/playwright-1.52.0-mac-arm64.zip

Fix

• Migrate import path: playwright-community/playwright-go → mxschmitt/playwright-go
• Bump version: v0.5200.0 → v0.6100.0
• Driver 1.61.0 is still available on Microsoft CDN

Changes

• lib/png/png.go — import path
• lib/xgif/xgif.go — import path
• d2cli/main.go — import path
• go.mod / go.sum — dependency update

Test

• d2 init-playwright succeeds
• Render to PNG/GIF/PDF works

Local Test

• Test init playwright

• Test render png to gif

[faisalaffan]

Hi @alixander, could you review this when you get a chance? This affects anyone trying to render non-SVG formats — with the deprecated module path and purged CDN drivers, d2 init-playwright is completely broken. Quite a few people are hitting this (seen in #2776 and elsewhere).

This is actually my first time contributing here. I discovered D2 through AI — tried PlantUML and Mermaid before, but neither came close to the completeness and beauty of D2, especially the animation support. Honestly mind-blowing to see a diagramming tool this polished. Huge respect for the work you've put into this.