Parse float8 to number

**Is your feature request related to a problem? Please describe.**

Currently, `float4` is being parsed as number, but `float8` is returned as string.
https://github.com/denodrivers/postgres/blob/main/query/decode.ts#L72

Based on node-pg-types, it seems that parsing `float8` with `parseFloat`, just like `float4`, should be fine.
https://github.com/brianc/node-pg-types/blob/master/lib/textParsers.js#L144

Is this difference intentional?
If there’s no specific reason for the difference, I think it would be clearer to align the behavior of the two.

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**