feat: upgrade to square-connect@4 and resolve greenkeeper suggestions

[ddimitrioglo]

Main changes

• [x] switch to square-connect@4
• [x] use @types/square-connect@4
• [x] improve error body parsing
• [x] fix tests
• [x] add test for calculate order endpoint

Greenkeeper

• [x] resolve https://github.com/goparrot/square-connect-plus/pull/10
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/11
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/12
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/13
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/14
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/15
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/17
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/20
• [x] resolve https://github.com/goparrot/square-connect-plus/pull/23

[coveralls]

Pull Request Test Coverage Report for Build 393600245

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.07%) to 98.917%

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Totals
Change from base Build 3c827b4e836e9b907717c77d435889dae0c7d591:	1.07%
Covered Lines:	139
Relevant Lines:	139

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💛 - Coveralls