marked.parseInline() returns string | Promise<string> even when { async: false }

**Marked version:** 10.0.0

**Describe the bug**
`marked.parseInline("markdown", { async: false })` returns a `string | Promise<string>`.

**Expected behavior**
`marked.parseInline("markdown", { async: false })` should return a `string`. This is already the case with `marked.parse("markdown", { async: false })`
