fix(config): Adds support for --offline flag to global add command

[fcrick]

Summary

Adds support for --offline flag to global add command.

Previously, this argument was ignored, resulting in issue #4913.

Test plan

Observe below how yarn currently goes to network when --offline is set with global add:

$ yarn --offline global add packagethatdoesnotexist@1.0.0
yarn global v1.16.0
[1/4] Resolving packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/packagethatdoesnotexist: Not found".
info If you think this is a bug, please open a bug report with the information provided in "C:\\Users\\fcrick\\AppData\\Local\\Yarn\\Data\\global\\yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/global for documentation about this command.

With fix, it now suggests running without --offline

$ node ./bin/yarn.js --offline global add packagethatdoesnotexist@1.0.0
yarn global v1.17.0-0
[1/4] Resolving packages...
error Couldn't find any versions for "packagethatdoesnotexist" that matches "1.0.0" in our cache (possible versions are ""). This is usually caused by a missing entry in the lockfile, running Yarn without the --offline flag may help fix this issue.
info Visit https://yarnpkg.com/en/docs/cli/global for documentation about this command.

Added test to verify this behavior.

[arcanis]

Thanks 👍