Add subcommand into filesize (#3987)

* Add subcommand `into filesize`

It's currently not possible to convert a number or a string containing a number
into a filesize. The only way to create an instance of filesize type today is
with a literal in nushell syntax. This commit adds the `into filesize`
subcommand so that file sizes can be created from the outputs of programs
producing numbers or strings, like standard unix tools.

There is a limitation with this - it doesn't currently parse values like `10 MB`
or `10 MiB`, it can only look at the number itself. If the desire is there, more
flexible parsing can be added.

* fixup! Add subcommand `into filesize`

* fixup! Add subcommand `into filesize`

crates/nu-command/tests/commands/mod.rs

@@ -25,6 +25,7 @@ mod headers;
 mod help;
 mod histogram;
 mod insert;
+mod into_filesize;
 mod into_int;
 mod keep;
 mod last;