Add support to convert strings to large integer and address `ScVal`s

[Shaptic]

In summary, this adds the following conversions:

• JS string -> ScVal with an ScAddress
• JS string -> ScVal with large integers such as Int128Parts (accomplished via XdrLargeInt)

For details, you can refer to the changes in test/unit/scval_test.js for new usage examples.

Closes #683.

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An unrelated change is to tests.yml which actually does prettier formatting before checking the diff :facepalm:. This is the cause of a handful of formatting changes (yarn fmt wasn't run in a previous PR).

[github-actions[bot]]

Size Change: +7.17 kB (0%)

Total Size: 3.13 MB

Filename	Size	Change
dist/stellar-base.js	2.3 MB	+5.3 kB (0%)
dist/stellar-base.min.js	835 kB	+1.87 kB (0%)

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