Move isomorphic-fetch to dev dependency

[penovicp]

Motivation and Resolution

Removes isomorphic-fetch as a production dependency because a fetch fallback should no longer be necessary in the majority of environments.

Usage related changes

Most users should not be affected. If there are some edge cases that still need a fetch fallback, a baseFetch override is enabled for the RpcProvider and RpcChannel classes through which it can be set.

Checklist:

• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Linked the issues which this PR resolves
• Documented the changes in code (API docs will be generated automatically)
• Updated the tests
• All tests are passing

[tabaktoni]

@dhruvkelawala @janek26 @yoga-braavos Can you check this one, will this still work for you ?

[avimak]

i'm good with the main change, but pls note the degradation related to specVersion; generally speaking, we should have it assigned locally next to this.chainId in any constructor