Fix parseSitemap and parseSitemapIndex uncatchable errors

[huntharo]

• parseSitemap and parseSitemapIndex both have an issue where errors in the _transform function are not catchable by the caller and instead result in an Unhandled error.
• The file that causes this problem was written with sitemap.js but it's not yet clear if the malformed file indicates that there is another bug to be fixed or if that is more indicative of a problem in my consumption of the library. More to follow on that if I find another issue.
• Added a test that fails before the fix and that passes after the fix

Before Fix - Output

The output below would result if parseSitemapIndex or parseSitemap were called on a malformed XML file that has an unescaped < in the text of a tag.

 FAIL  tests/sitemap-index-parser.test.ts
  ● parseSitemapIndex › rejects malformed file

    Unhandled error. (Error: Unencoded <
    Line: 10
    Column: 25
    Char: <

      127 |     // eslint-disable-next-line @typescript-eslint/ban-ts-comment
      128 |     // @ts-ignore
    > 129 |     this.saxStream.write(data, encoding);
          |                    ^
      130 |     callback();
      131 |   }
      132 | }

      at error (node_modules/sax/lib/sax.js:651:10)
      at strictFail (node_modules/sax/lib/sax.js:677:7)
      at SAXParser.write (node_modules/sax/lib/sax.js:1080:13)
      at SAXStream.write (node_modules/sax/lib/sax.js:239:18)
      at XMLToSitemapIndexStream._transform (lib/sitemap-index-parser.ts:129:20)
      Line: 10
      Column: 25
      Char: <
      at error (node_modules/sax/lib/sax.js:651:10)
      at strictFail (node_modules/sax/lib/sax.js:677:7)
      at SAXParser.write (node_modules/sax/lib/sax.js:1080:13)
      at SAXStream.write (node_modules/sax/lib/sax.js:239:18)
      at XMLToSitemapIndexStream._transform (lib/sitemap-index-parser.ts:129:20)
      at SAXParser.SAXStream._parser.onerror (node_modules/sax/lib/sax.js:194:10)
      at emit (node_modules/sax/lib/sax.js:624:35)
      at error (node_modules/sax/lib/sax.js:653:5)
      at strictFail (node_modules/sax/lib/sax.js:677:7)
      at SAXParser.write (node_modules/sax/lib/sax.js:1080:13)
      at SAXStream.write (node_modules/sax/lib/sax.js:239:18)
      at XMLToSitemapIndexStream._transform (lib/sitemap-index-parser.ts:129:20)

[derduher]

Thank you for the quality submission