feat: warn web-ext v7 users about upcoming changes to the sign command

[willdurand]

Fixes #2877

---

This will look like this:

❯ ./bin/web-ext.js sign --api-key="foo" --api-secret="bar"
Applying config file: ./package.json
IMPORTANT: web-ext v8 will introduce a new AMO submission API for signing and you might need to take actions when upgrading from v7 to v8. You can test this upcoming change now by specifying `--use-submission-api` to the `sign` command. If you want to keep using the API you are using now (via "sign-addon"), you must stay on v7.

[...]

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3bc0548) 99.53% compared to head (57acda7) 99.53%.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           minor-releases-7.x.x    #2879   +/-   ##
=====================================================
  Coverage                 99.53%   99.53%           
=====================================================
  Files                        32       32           
  Lines                      1705     1707    +2     
=====================================================
+ Hits                       1697     1699    +2     
  Misses                        8        8           

Files Changed	Coverage Δ
src/cmd/sign.js	100.00% <100.00%> (ø)

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