docs: fix 17 typos across README and proposals

[Avicennasis]

Summary

Fix 17 typos and grammar issues across 8 files:

• README.md: "markout" → "mark out"
• APPLICATION-GUIDE.md: "accomodated" → "accommodated", "accomodating" → "accommodating", remove duplicate "here"
• proposals/servo-webgpu.md: "a an" → "an", "can be ran" → "run", "Vulkan, D3D12, and Vulkan" → "Metal" (duplicate), "test suit" → "suite", "will to learn" → "willingness" (×2)
• proposals/webrender-gfx-opengl.md: "will to learn" → "willingness"
• proposals/devtools-overflow-debugging.md: "developer tool" → "tools", "elementsd" → "elements"
• proposals/common-voice-kiosk.md: "desings" → "designs"
• proposals/extension-activity-monitor.md: "expore" → "explore"
• proposals/devtools-server-sent-events.md: "Javascript" → "JavaScript", missing article "a"

No functional changes — documentation only.