fix(connext): avoid scientific notation for amount

[sangaman]

This changes the way we convert numbers to strings when building requests to send to the Connext client by using toLocaleString instead of toString to avoid representing large numbers with scientific notation.

This should help with issues we've faced using toknes that deal with larger quantities like DAI.

An example of how toLocaleString compares with toString as well as bigint:

const x = 1725000000000000000000; // 1725 DAI in wei units
const bigX = 1725000000000000000000n;

console.log(x.toString());                                          // 1.725e+21
console.log(x.toLocaleString('fullwide', { useGrouping: false }));  // 1725000000000000000000
console.log(bigX.toString());                                       // 1725000000000000000000

const y = x + 1;
console.log(y.toLocaleString('fullwide', { useGrouping: false }));  // 1725000000000000000000
// but we will never be making payments with wei-level precision, xud only uses up to 8 digits to right of decimal point
// the approach above gives us up to 16 digits of precision, which is on the order of 100 million + 8 decimal places of precision

[ghost]

LGTM 👍