fix: unnecessary down-leveled declarations written

The test that assesses whether down-leveled declarations are necessary
or not accounted for the possible presence of duplicated references to
`node` types being injected by the TypeScript compiler
(microsoft/TypeScript#48143), however in some cases the reference is
present more than twice, which was not correctly accounted for;
resulting in emission of redundant down-level declarations.

Author: RomainMuller

src/downlevel-dts.ts

@@ -185,7 +185,7 @@ function semanticallyEqualDeclarations(left: string, right: string): boolean {
     // TypeScript may emit duplicated reference declarations... which are absent from Downlevel-DTS' output...
     // https://github.com/microsoft/TypeScript/issues/48143
     const REFERENCES_TYPES_NODE = '/// <reference types="node" />';
-    if (normalized.startsWith(`${REFERENCES_TYPES_NODE}\n${REFERENCES_TYPES_NODE}`)) {
+    while (normalized.startsWith(`${REFERENCES_TYPES_NODE}\n${REFERENCES_TYPES_NODE}`)) {
       normalized = normalized.slice(REFERENCES_TYPES_NODE.length + 1);
     }