fix: normalize category elements with attributes to plain strings

[pratik-desgn]

Fixes #289.

index.d.ts declares Item.categories as string[], but xml2js returns a {_, $} object for any <category> element that has attributes (e.g. domain="..."), since item.categories was being set directly from the raw xml2js output with no normalization:

if (xmlItem.category) item.categories = xmlItem.category;

For a feed like:

<category domain="changelog-type">Release</category>
<category domain="changelog-label">copilot</category>

categories came out as:

[
  { _: 'Release', '$': { domain: 'changelog-type' } },
  { _: 'copilot', '$': { domain: 'changelog-label' } }
]

instead of ['Release', 'copilot'] as declared.

Fix

Extract the text value the same way item.guid already does a few lines above:

if (xmlItem.category) {
  item.categories = xmlItem.category.map((category) => (category && category._) || category);
}

This normalizes both shapes (plain string categories and {_, $} categories-with-attributes) to plain strings, matching the declared type.

Testing

• Added test/input/item-category-with-attrs.rss + golden output, reproducing the exact shape from the issue (mixed attributed/plain <category> tags) — confirmed this test fails on the old code with the exact reported shape, passes with the fix.
• Regenerated the golden output for 2 pre-existing fixtures (heraldsun, guardian) that happened to already contain <category domain="..."> elements and had the buggy {_, $} shape baked into their snapshots — diffed to confirm the change is scoped to exactly the categories field, nothing else.
• Full suite passes (37 tests).