feat: add `isTestnet` property to `ArbitrumNetwork` #1448

Job	Run time
Install on Node.js v18	54s
Install on Node.js v18	16s
Install on Node.js v18	16s
Install on Node.js v20	48s
Install on Node.js v20	9s
Install on Node.js v20	18s
Lint on Node.js v18	8s
Lint on Node.js v18	8s
Lint on Node.js v18	6s
Test (Unit) on Node.js v18	8s
Test (Unit) on Node.js v18	5s
Test (Unit) on Node.js v18	4s
Test (Integration) on Node.js v18	9m 49s
Test (Integration) on Node.js v18	38s
Test (Integration) on Node.js v18	49s
Test (Unit) on Node.js v20	5s
Test (Unit) on Node.js v20	8s
Test (Unit) on Node.js v20	9s
Lint on Node.js v20	5s
Lint on Node.js v20	8s
Lint on Node.js v20	11s
Audit on Node.js v18	6s
Audit on Node.js v18	5s
Audit on Node.js v18	8s
Test (Integration) on Node.js v18 with L3	10m 51s
Test (Integration) on Node.js v18 with L3	38s
Test (Integration) on Node.js v18 with L3	49s
Audit on Node.js v20	8s
Audit on Node.js v20	5s
Audit on Node.js v20	6s
Test (Integration) on Node.js v20	9m 46s
Test (Integration) on Node.js v20	39s
Test (Integration) on Node.js v20	47s
Test (Integration) on Node.js v20 with L3	10m 45s
Test (Integration) on Node.js v20 with L3	38s
Test (Integration) on Node.js v20 with L3	47s
Test (Integration) on Node.js v18 with L3 with custom gas token	11m 15s
Test (Integration) on Node.js v18 with L3 with custom gas token	37s
Test (Integration) on Node.js v18 with L3 with custom gas token	46s
Test (Integration) on Node.js v20 with L3 with custom gas token	11m 27s
Test (Integration) on Node.js v20 with L3 with custom gas token	39s
Test (Integration) on Node.js v20 with L3 with custom gas token	49s
	1h 17m 13s

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