feat: environment flag for default file size style

Something like an `EZA_SIZE_STYLE` with the possible values `binary`, `decimal`, and `raw` for selecting how to display file sizes by default.

The values `binary`, and `raw` are equivalent to the options `-b/--binary` and `-B/--bytes`, respectively. For the value `decimal`, I propose adding a `--decimal` CLI flag. This would allow the end user to override the default file size style whenever they wanted. Given that the decimal is the default style, I see no need to add a short flag for it as that option will be rarely used.

I think that this is a good first issue and I think I can implement it but I would like to hear from the project's authors/maintainers first.

Start by tracing the existing -b/--binary and -B/--bytes option handling, then identify where environment-based defaults are read. Define the precedence between EZA_SIZE_STYLE values and the proposed --decimal override, and verify that binary, decimal, and raw produce the documented default size styles.

Written by the indexing model from the issue text.