fix: do the math ourselves (jquense#2220)

Remove requirement for SASS math plugin by doing the math ourselves

.size-snapshot.json

@@ -24,26 +24,26 @@
     }
   },
   "react-big-calendar.js": {
-    "bundled": 1589216,
-    "minified": 446843,
-    "gzipped": 137873
+    "bundled": 1626986,
+    "minified": 458062,
+    "gzipped": 141015
   },
   "react-big-calendar.min.js": {
-    "bundled": 285015,
-    "minified": 283564,
-    "gzipped": 89629
+    "bundled": 296359,
+    "minified": 294893,
+    "gzipped": 92856
   },
   "react-big-calendar.esm.js": {
-    "bundled": 200727,
-    "minified": 93588,
-    "gzipped": 24341,
+    "bundled": 204084,
+    "minified": 95141,
+    "gzipped": 24761,
     "treeshaked": {
       "rollup": {
-        "code": 66887,
-        "import_statements": 1823
+        "code": 68272,
+        "import_statements": 1772
       },
       "webpack": {
-        "code": 70478
+        "code": 71884
       }
     }
   }

src/sass/variables.scss

@@ -1,5 +1,3 @@
-@use "sass:math";
-
 $out-of-range-color: lighten(#333, 40%) !default;
 $out-of-range-bg-color: lighten(#333, 70%) !default;
 
@@ -8,7 +6,8 @@ $cell-border: #ddd !default;
 
 $border-color: #ccc !default;
 
-$segment-width: percentage(math.div(1, 7)) !default;
+// Each calendar segment is 1/7th.
+$segment-width: 0.14286% !default;
 
 $time-selection-color: white !default;
 $time-selection-bg-color: rgba(0, 0, 0, 0.5) !default;