Allow checking for WM support before window's load event

[raducristianpopa]

Context

With the current extension, checking for WM support is possible only after window's load event. The page should be able to check for WM support before window's load event is triggered.

• WM Spec: Checking if Web Monetization is supported

Todos

• Inject polyfill on document_start (before window.onload) (--- ADDRESSED IN Trigger load event after checking the wallet address #43 )
• Details for MV2 and MV3: CustomEvent properties inside detail won't match spec #9 (--- ADDRESSED IN [Monetization flow - Content] Dispatch Monetization event #163)

[tselit]

Addressed in #203.
Ready to close.