Add formatGwei

[zbcoding]

Add formatGwei which works similarly to formatEther. Gwei is commonly used for units of eth gas fees.

[ricmoo]

How is this different than using formatUnits(value, "gwei")?

[zbcoding]

How is this different than using formatUnits(value, "gwei")?

and there's formatEther(value) too, which is simply formatUnits(value, 18) or formatUnits(value, "ether")

So same idea, a special function could be useful since gwei is commonly used . Perhaps too much extra code to add a function for each value in the unit index - but since ether and gwei are the most used having functions for them makes sense