Introduce method for formatting file size

[abjerner]

Since formatting a file size (byte count) is something we do over and over again, it's a good candiate for adding to Skybrud.Essentials.

It's worth noticing that there are both a decimal format (1000 bytes is 1 KB), and a binary format (1024 bytes is 1 KiB), but also that KB is commonly used instead of KiB to refer to 1024 bytes.

Usage could be something like:

Implicit (binary, but with decimal units - technically incorrect, but commonly used)

// Returns "1 KB"
string formatted = StringUtils.FormatFileSize(1024);

Default (binary, but with decimal units - technically incorrect, but commonly used)

// Returns "1 KB"
string formatted = StringUtils.FormatFileSize(1024, FileSizeFormat.Default);

Binary

// Returns "1 KiB"
string formatted = StringUtils.FormatFileSize(1024, FileSizeFormat.Binary);

or:

// Returns "1 KiB"
string formatted = StringUtils.FormatFileSize(1024, FileSizeFormat.Kibi);

Decimal

// Returns "1 KB"
string formatted = StringUtils.FormatFileSize(1000, FileSizeFormat.Decimal);

or:

// Returns "1 KB"
string formatted = StringUtils.FormatFileSize(1000, FileSizeFormat.Kilo);