…r ints (nushell#11724)

# Description

This PR changes `into int` and `into filesize` so that they allow
thousands separators.

### Before
```nushell
❯ '1,000' | into filesize
Error: nu::shell::cant_convert

  × Can't convert to int.
   ╭─[entry nushell#1:1:1]
 1 │ '1,000' | into filesize
   · ───┬───
   ·    ╰── can't convert string to int
   ╰────

❯ '1,000' | into int
Error: nu:🐚:cant_convert

  × Can't convert to int.
   ╭─[entry nushell#2:1:1]
 1 │ '1,000' | into int
   ·           ────┬───
   ·               ╰── can't convert string to int
   ╰────
  help: string "1,000" does not represent a valid integer
```
### After
```nushell
❯ '1,000' | into filesize
1.0 KB
❯ '1,000' | into int
1000
```

This works by getting the system locale and from that, determining what
the thousands separator is. So, hopefully, this will work across
locales.
# User-Facing Changes
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