fix(javascript): correct escape for markdown in js (#4381)

* test: add tests

* fix(javascript): correct escape for markdown in js

* test: add trailing newline

src/language-js/embed.js

@@ -164,7 +164,10 @@ function embed(path, print, textToDoc /*, options */) {
             (parentParent.tag.name === "md" ||
               parentParent.tag.name === "markdown")))
       ) {
-        const text = parent.quasis[0].value.cooked;
+        const text = parent.quasis[0].value.raw.replace(
+          /((?:\\\\)*)\\`/g,
+          (_, backslashes) => "\\".repeat(backslashes.length / 2) + "`"
+        );
         const indentation = getIndentation(text);
         const hasIndent = indentation !== "";
         return concat([
@@ -207,7 +210,7 @@ function escapeBackticks(doc) {
 
     currentDoc.parts.forEach(part => {
       if (typeof part === "string") {
-        parts.push(part.replace(/`/g, "\\`"));
+        parts.push(part.replace(/(\\*)`/g, "$1$1\\`"));
       } else {
         parts.push(part);
       }

tests/multiparser_js_markdown/__snapshots__/jsfmt.spec.js.snap

@@ -69,6 +69,35 @@ markdown\\\`
 
 `;
 
+exports[`escape.js 1`] = `
+markdown\`
+  const cssString = css\\\`
+    background-color: \\$\\{color('base')\\}
+  \\\`;
+\`
+
+markdown\`
+  - \\\`
+  - \\\\\\\`
+  - \\\\\\\\
+  - \\$
+  - \\u1234
+\`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+markdown\`
+  const cssString = css\\\`background-color: \\$\\{color('base')\\}\\\`;
+\`;
+
+markdown\`
+  * \\\`
+  * \\\\\\\`
+  * \\\\\\\\
+  * \\$
+  * \\u1234
+\`;
+
+`;
+
 exports[`markdown.js 1`] = `
 export default function ReadMe() {
     return md\`

tests/multiparser_js_markdown/escape.js

@@ -0,0 +1,13 @@
+markdown`
+  const cssString = css\`
+    background-color: \$\{color('base')\}
+  \`;
+`
+
+markdown`
+  - \`
+  - \\\`
+  - \\\\
+  - \$
+  - \u1234
+`