Add battery capacity support on FreeBSD (#988)

## What are the changes about?

So far ly only supported showing battery capacity on Linux. This adds support for FreeBSD as well using `sysctlbyname()` instead of reading from `/sys/...`

## What existing issue does this resolve?

No battery shown on FreeBSD.

_Replace this with a reference to an existing issue, or N/A if there is none_

## Pre-requisites

- [x] I have tested & confirmed the changes work locally
- [x] I have run `zig fmt` throughout my changes

Co-authored-by: AnErrupTion <anerruption+codeberg@disroot.org>
Co-authored-by: AnErrupTion <anerruption@disroot.org>
Reviewed-on: https://codeberg.org/fairyglade/ly/pulls/988
Reviewed-by: AnErrupTion <anerruption+codeberg@disroot.org>

ly-core/src/interop.zig

@@ -14,6 +14,9 @@ pub const utmp = @import("utmp");
 // Exists for X11 support only
 pub const xcb = @import("xcb");
 
+// FreeBSD only
+pub const sysctl = @import("sysctl");
+
 pub const TimeOfDay = struct {
     seconds: i64,
     microseconds: i64,