make small IconButton symmetrical

.changeset/light-brooms-count-1.md

@@ -0,0 +1,5 @@
+---
+'@itwin/itwinui-react': patch
+---
+
+Updated the size of `<IconButton styleType="borderless" size="small">` so that it has symmetrical dimensions (square).

.changeset/light-brooms-count.md

@@ -0,0 +1,5 @@
+---
+'@itwin/itwinui-react': patch
+---
+
+Updated the size of small borderless icon-only button, so that it has symmetrical dimensions (square).