check if wasm and zkey exist

[s-ekai]

fix: #49

description:

• I will check if there are files at wasmFilePath and finalZkeyPath.
• The path can be either a local file path or a URL.
• There are two environments at runtime: the browser environment and the Node.js environment. During the browser environment, you cannot access the path to a local file.
• I am using the window and process variables to determine if it's a browser environment.

[mhchia]

@s-ekai Awesome work! Could you run npm run format to fix the lint?

[s-ekai]

@mhchia
Thank you very much, I ran npm run format!

[mhchia]

@s-ekai Great work. Thanks a lot for your contribution 😄

[s-ekai]

@mhchia Thank you very much for checking!