feat: expand tildes in config theme path to HOME

When loading a theme from a path that includes a tilde:
```
theme="~/.cache/wal/colors-ghostty"
```
currently fails with the following error:
```
cannot include path separators unless it is an absolute path
```

This PR tries to expand the ~ of the path. If there is no ~
or expansion fails, it falls back to the unexpanded value.

src/config/theme.zig

@@ -5,6 +5,8 @@ const internal_os = @import("../os/main.zig");
 const cli = @import("../cli.zig");
 const global = @import("../global.zig");
 
+const log = std.log.scoped(.config);
+
 /// Location of possible themes. The order of this enum matters because it
 /// defines the priority of theme search (from top to bottom).
 pub const Location = enum {
@@ -113,12 +115,40 @@ pub const LocationIterator = struct {
 /// will be added to the list and null will be returned.
 pub fn open(
     arena_alloc: Allocator,
-    theme: []const u8,
+    theme_: []const u8,
     diags: *cli.DiagnosticList,
 ) error{ OutOfMemory, Unexpected }!?struct {
     path: []const u8,
     file: std.Io.File,
 } {
+    var buf: [std.fs.max_path_bytes]u8 = undefined;
+    const theme = expanded: {
+        if (!std.mem.startsWith(u8, theme_, "~/"))
+            break :expanded theme_;
+
+        var environ_map = global.environMap() catch |err| {
+            log.warn(
+                "error getting environment map when expanding theme path={s}: {}",
+                .{ theme_, err },
+            );
+            break :expanded theme_;
+        };
+        defer environ_map.deinit();
+
+        break :expanded internal_os.expandHome(
+            global.io(),
+            &environ_map,
+            theme_,
+            &buf,
+        ) catch |err| {
+            log.warn(
+                "error expanding home directory for theme path={s}: {}",
+                .{ theme_, err },
+            );
+            break :expanded theme_;
+        };
+    };
+
     // Absolute themes are loaded a different path.
     if (std.fs.path.isAbsolute(theme)) {
         const file: std.Io.File = try openAbsolute(