Contains a few improvements to rough edges around theno-default-alt-text rule that I noticed while working on myMarkdown checker browser extension:

• Made the description more succinct: "Images should have meaningful alternative text (alt text)" instead of "Images should set meaningful alternative text (alt text), and not use the macOS default screenshot filename orImage." This shorter description is consistent with the default Markdownlint rules
• Removed thedetail field from failures, since it was just a copy of the image's alt text. This was distracting and not very helpful - the detail field is only really useful for showing hints on how to resolve the issue. For example, a useful detail forMD001 looks like: "Expected: h2; Actual: h3"
• Added arange field that only covers the alt text so we don't have to error the entire line. This helps draw attention to the specific issue at hand and helps writers figure out specifically where to put better alt text. This required using regular expressions for all checking (instead of only for HTML), because the default Markdown parser tokens don't have ranges

Note: The use of thed flag in the regular expressions (which is necessary in order to get the range indexes) means we can only now support later versions of Node since this is an ES2022 feature.