feat: Resolve deprecation notices (global built-in functions) (#72)

## Description
There are a large number of deprecation notices being shown due to
incoming changes in SASS. This change addresses the notices relating
to global built-in functions.

## Issue
https://jira.dev.bbc.co.uk/browse/GEL

Author: randombitsUK

_typography.scss

@@ -1,3 +1,5 @@
+@use 'sass:map';
+
 ///*------------------------------------*\
 //    # GEL TYPOGRAPHY
 //\*------------------------------------*/
@@ -20,7 +22,7 @@ $enhanced: true !default;
 
 // If larger font sizes enable, merge them with gel-type-settings
 @if $gel-type-enable--larger-type-sizes {
-    $gel-type-settings: map-merge($gel-type-settings, $gel-larger-font-sizes);
+    $gel-type-settings: map.merge($gel-type-settings, $gel-larger-font-sizes);
 }
 
 // Output the typography styles for the 4 groups

lib/_settings.scss

@@ -1,3 +1,5 @@
+@use 'sass:meta';
+
 ///*------------------------------------*\
 //    # GEL TYPOGRAPHY - SETTINGS
 //\*------------------------------------*/
@@ -412,7 +414,7 @@ $gel-larger-font-sizes: (
 // Adds the typography specific breakpoints to the Sass MQ list
 // of breakpoints
 //
-@if (mixin-exists(mq)) {
+@if (meta.mixin-exists(mq)) {
     @include mq-add-breakpoint(gel-bp-type-b, 320px);
     @include mq-add-breakpoint(gel-bp-type-c, 600px);
 } @else {

lib/_tools.scss

@@ -1,3 +1,5 @@
+@use 'sass:map';
+
 ///*------------------------------------*\
 //    # GEL TYPOGRAPHY - TOOL
 //\*------------------------------------*/
@@ -44,10 +46,10 @@
 
         @if map-has-key($source, $key) {
             $current: map-get($source, $key);
-            $new: ($key: map-merge($current, $value));
+            $new: ($key: map.merge($current, $value));
         }
 
-        $source: map-merge($source, $new);
+        $source: map.merge($source, $new);
     }
 
     @return $source;

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "gel-typography",
-  "version": "5.1.2",
+  "version": "5.2.0",
   "description": "A flexible code implementation of the GEL Typography",
   "main": "_typography.scss",
   "scripts": {
@@ -42,6 +42,6 @@
     "sass-mq": "6.0.0"
   },
   "dependencies": {
-    "gel-sass-tools": "3.3.0"
+    "gel-sass-tools": "3.4.0"
   }
 }