fonts: add IBM Plex Serif (#65690)

README

@@ -376,6 +376,13 @@ copyright notices:
   #
   # https://github.com/rsms/inter
 
+* IBM Plex Serif
+  # Copyright © 2017 IBM Corp. with Reserved Font Name "Plex".
+  #
+  # This Font Software is licensed under the SIL Open Font License, Version 1.1.
+  #
+  # https://www.fontsquirrel.com/fonts/ibm-plex
+
 ~~~~
 
 Some theme variant files have their own specific licenses; refer to

static/includes/fonts/_ibm-plex-serif.scss

@@ -0,0 +1,24 @@
+@mixin ibm-font($type, $weight, $style) {
+	@font-face {
+		font-family: 'IBM Plex Serif';
+		src: url('/static/fonts/IbmPlexSerif/IBMPlexSerif-#{$type}.woff2') format('woff2'),
+		     url('/static/fonts/IbmPlexSerif/IBMPlexSerif-#{$type}.woff') format('woff');
+		font-weight: $weight;
+		font-style: $style;
+	}
+}
+
+@mixin ibm-font-pair($type, $weight) {
+	@include ibm-font($type, $weight, normal);
+	@include ibm-font('#{$type}Italic', $weight, italic);
+}
+
+@include ibm-font-pair('Thin', 100);
+@include ibm-font-pair('ExtraLight', 200);
+@include ibm-font-pair('Light', 300);
+@include ibm-font-pair('Medium', 500);
+@include ibm-font-pair('SemiBold', 600);
+@include ibm-font-pair('Bold', 700);
+@include ibm-font-pair('Text', 400);
+@include ibm-font('Regular', 400, normal);
+@include ibm-font('Italic', 400, italic);