fix: Update function call (#65)

## Description
Resolves an issue the rem is called when handling mulitple values, rather
than the expected toRem function. At the same time a deprecation warning
has been addressed when using the sass function `append`.

Author: randombitsUK

lib/tools/_rem.scss

@@ -1,3 +1,4 @@
+@use 'sass:list';
 @use 'sass:math';
 @use 'sass:meta';
 @use '../settings/globals';
@@ -33,7 +34,7 @@ $gel-tools-rem-enable--fallback: true !default;
     $result: ();
 
     @each $e in $value {
-      $result: append($result, rem($e, $baseline));
+      $result: list.append($result, toRem($e, $baseline));
     }
 
     @return $result;

test/__snapshots__/rem.test.js.snap

@@ -1,5 +1,11 @@
 // Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
 
+exports[`rem() > function > should return multiple values 1`] = `
+".multiples {
+  padding: 1rem 1.5rem;
+}"
+`;
+
 exports[`rem() > function > should return pixel values converted to rem 1`] = `
 ".demo-1 {
   font-size: 1rem;

test/data/rem/multiples.scss

@@ -0,0 +1,6 @@
+@use '../../../sass-tools';
+
+.multiples {
+  // Passing a list to toRem triggers the buggy code path
+  padding: sass-tools.toRem(16px 24px);
+}

test/rem.test.js

@@ -5,6 +5,12 @@ describe('rem()', () => {
 
       expect(css).toMatchSnapshot();
     });
+
+    it('should return multiple values', async () => {
+      const css = await global.compileCSS('test/data/rem/multiples.scss');
+
+      expect(css).toMatchSnapshot();
+    });
   });
 
   describe('mixin', () => {