[CDTOOL-1255] Move dictionary-entry commands under service

[rcaril]

Change summary

This PR moves the dictionary-entry command under the service command. A alias for fastly dictionary-entry is added here to prevent breaking changes.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

 make test TEST_ARGS="-run TestDictionaryItemDescribe ./pkg/commands/service/dictionaryentry"
ok      github.com/fastly/cli/pkg/commands/service/dictionaryentry      1.058s
make test TEST_ARGS="-run TestDictionaryItemsList ./pkg/commands/service/dictionaryentry"
ok      github.com/fastly/cli/pkg/commands/service/dictionaryentry      1.064s
make test TEST_ARGS="-run TestDictionaryItemCreate ./pkg/commands/service/dictionaryentry"
ok      github.com/fastly/cli/pkg/commands/service/dictionaryentry      1.056s
make test TEST_ARGS="-run TestDictionaryItemUpdate ./pkg/commands/service/dictionaryentry"
ok      github.com/fastly/cli/pkg/commands/service/dictionaryentry      1.073s
make test TEST_ARGS="-run TestDictionaryItemDelete ./pkg/commands/service/dictionaryentry"
ok      github.com/fastly/cli/pkg/commands/service/dictionaryentry      1.056s

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

None, an alias was added to prevent breaking changes.

[kpfleming]

This will need to be rebased before it can be reviewed and merged.