fix: keep lovelace amounts that do not fit in a JS number

[SashaMIT]

Summary

MeshValue.toJSON summed asset quantities with Number. A JS number cannot hold every lovelace amount. 9007199254740993 lovelace (about 9 billion ADA, under the 45 billion supply) came back as 9007199254740992.

Quantities are now summed as bigint. A value that still fits in a JS number stays a number, so existing JSON output is unchanged. A value that does not fit is passed to integer() as a bigint.

Test plan

• On tip, toJSON of lovelace 9007199254740993 returned the number 9007199254740992.
• After the change, that quantity is the bigint 9007199254740993, and 1000000 lovelace is still the number 1000000.
• npm test in @meshsdk/common was not run here. The new case is in test/data/value/convertor.test.ts.

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