fix: parse plugin repo metadata as JSON5

[puneetdixit200]

Fixes #4084.

Summary

• Decode installed plugin repo.json metadata with json5, matching the plugin installer and channel/repository decoding paths.
• Add a regression test covering a repo.json payload with a JSON5 comment.

Tests

• Red before fix: go test ./internal/config -run TestNewPluginInfoAllowsJSON5Comments -count=1 failed with invalid character '/' looking for beginning of value
• go test ./internal/config -run TestNewPluginInfoAllowsJSON5Comments -count=1
• go test ./internal/config -count=1
• go test ./...
• git diff --check