fix: Codesign issue when app name has spaces

[lcheunglci]

I created an issue: https://github.com/electron/electron-osx-sign/issues/237

I believe this was caused by not having the escape character when the codesign command string was being constructed; therefore, causing the codesign command to fail when codesigning the executable e.g. App name.app/Contents/MacOS/App name

I was able to successfully codesign with electron-osx-sign with the change. I hope this resolves issues that others may have run into because their electron app product name has spaces.

[lcheunglci]

I was merging the newer changes into my fork since I've made this PR. I see that the sign.js has been replaced with sign.ts. While, I was re-applying the changes from merging, I was going to improve string replace for the space character with the escape character to _opts.app using regex (https://stackoverflow.com/questions/44738314/escape-a-space-in-a-file-path-in-node-js) but realized, that with typescript the _opts i.e. SignOption is read-only, so this approach won't work. Perhaps, the more appropriate way is to update the documentation in the README.md that app with space is not supported and throw an exception during the validation in the util.ts in the validateOptsApp so it won't run into an error during the mac codesign process. Let me know if that approach is preferred and I'll make a commit to the README.md and util.ts instead.