Hello and thank you for your interest!
Could you please suggest some starter issues to help me get familiar with the project and Swift codebase?
You could start by looking at "good first issue" labeled issues.
Could you recommend some resources to learn more about macros and property wrappers?
I think you should start with property wrappers proposal, there are multiple proposals for macros as well starting from expression macros. Understanding current capabilities of property wrappers and macros is essential.
Is building a demo for this project an important requirement before submitting the proposal, or is it just a "nice to have" addition to demonstrate my understanding?
No demo required.
Would not working on Xcode and macOS be a limitation for contributing to the Swift compiler repository?
No, Xcode and macOS are not a requirement. The compiler could be developed on Linux and Windows as well.
Can I start working on the project design and a small demo for it without making any previous PRs on Swift repo ?
I'd suggest you start with some "good first issue" and/or property wrapper bugs and go from there. The project spans multiple areas of the compiler such as semantic analysis and SIL generation so in order to prepare a strong proposal you'd have to understand how some of that works in regards to property wrapper support.