Avoid introducing linebreaks in template interpolations (#15209)

changelog_unreleased/javascript/15209.md

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+#### Avoid introducing linebreaks in template interpolations (#15209 by @bakkot)
+
+In a template string like
+
+<!-- prettier-ignore -->
+```js
+`this is a long message which contains an interpolation: ${format(data)} <- like this`;
+```
+
+avoid adding a linebreak when formatting the expression unless one is already present or it's unavoidable due to e.g. a nested function. Previously a linebreak could be introduced whenever some interpolation in the template was sufficiently "not simple":
+
+<!-- prettier-ignore -->
+```js
+`this is a long message which contains an interpolation: ${format(
+  data,
+)} <- like this`;
+```
+
+Now it will instead be left alone.
+
+If a linebreak is already present within the `${...}`, format as normal.