feat: add `isTestnet` property to `ArbitrumNetwork` #1452

Job	Run time
Install on Node.js v18	13s
Install on Node.js v20	8s
Test (Unit) on Node.js v18	1m 10s
Test (Integration) on Node.js v18	27m 41s
Lint on Node.js v18	53s
Audit on Node.js v18	15s
Test (Unit) on Node.js v20	1m 9s
Test (Integration) on Node.js v18 with L3	20m 15s
Lint on Node.js v20	58s
Audit on Node.js v20	17s
Test (Integration) on Node.js v20	27m 32s
Test (Integration) on Node.js v20 with L3	20m 13s
Test (Integration) on Node.js v18 with L3 with custom gas token	22m 2s
Test (Integration) on Node.js v20 with L3 with custom gas token	22m 2s
	2h 24m 48s

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