Common: consistent `GasPrices` naming

[acolytec3]

As discovered in #3035, the casing of the GasPrices subkeys in common/src/eips.ts is not consistent. This should be fixed and then simplify the code in places like in the getBaseFee helper where we call toLowerCase to remove this additional check (which in theory should also provide a small performance gain).

[holgerd77]

Just to add that this is a breaking change since these names are part of the public Common API, so we can't do these updates right now.

[holgerd77]

Have given this a more neutral title (I somewhat do like the camel case naming, so I would like to at least give this one additional round of thought what we do here (all lowercase vs consistent camel case). No dramatically strong opinion though. 😋

Will also add to the new breaking release tracking issue I created #3216