Phemex setMarginMode
Phemex setMarginMode

Added setMarginMode to Phemex:
Cross is selected by setting leverage to 0, Isolated is 1x to 100x
```
node examples/js/cli phemex setMarginMode cross BTC/USD:BTC

phemex.setMarginMode (cross, BTC/USD:BTC)
2022-05-10T04:40:39.504Z iteration 0 passed in 552 ms

{ code: '0', msg: '', data: 'OK' }
```

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