fix(native): measure em against the font size each element computes

src/__tests__/compiler/font-size.test.ts

@@ -0,0 +1,52 @@
+import { compileWithAutoDebug } from "react-native-css/jest";
+
+const parentRelative = (size: unknown) => [{}, "fontSize", size, 1];
+
+const rulesOf = (css: string) =>
+  compileWithAutoDebug(`.my-class { ${css} }`)
+    .stylesheet()
+    .s?.find((rule) => rule[0] === "my-class")?.[1];
+
+// prettier-ignore
+test.each([
+  ["font-size: 2em", parentRelative([{}, "em", 2, 1])],
+  ["font-size: 150%", parentRelative([{}, "em", 1.5, 1])],
+  ["font-size: 33.333%", parentRelative([{}, "em", 33.333 / 100, 1])],
+  ["font: 150% Arial", parentRelative([{}, "em", 1.5, 1])],
+  ["font-size: var(--size)", parentRelative([{}, "var", "size", 1])],
+])("%s is computed against the parent's font size and published as the em", (css, fontSize) => {
+  const [rule] = rulesOf(css) ?? [];
+
+  expect(rule?.d).toContainEqual([fontSize, "fontSize", 1]);
+  expect(rule?.v).toContainEqual(["__rn-css-em", fontSize]);
+  expect(rule?.dv).toBe(1);
+});
+
+test("an absolute font size is static and published as the em", () => {
+  expect(rulesOf("font-size: 10px")).toStrictEqual([
+    {
+      d: [{ fontSize: 10 }],
+      s: [1, 1],
+      v: [["__rn-css-em", 10]],
+    },
+  ]);
+});
+
+test("a rule reading em reads the inherited em channel", () => {
+  expect(rulesOf("width: 2em")).toStrictEqual([
+    {
+      d: [[[{}, "em", 2, 1], "width", 1]],
+      dv: 1,
+      s: [1, 1],
+    },
+  ]);
+});
+
+test("a font size crosses the wire unchanged", () => {
+  const stylesheet = compileWithAutoDebug(
+    `.a { font-size: 2em; } .b { font-size: 150%; } .c { font: 150% Arial; } .d { font-size: var(--size); }`,
+  ).stylesheet();
+  const wire = JSON.stringify(stylesheet);
+
+  expect(JSON.parse(wire)).toStrictEqual(stylesheet);
+});