Add blog utilities and tag pages

Introduce formatBlogDate, slugifyTag and getTagHref in src/lib/blog.ts
and
a reading time helper getReadingTime in src/lib/readingTime.ts. Update
blog index and post pages to display ISO dates, reading times and link
tags to /blog/tag/<slug>. Add a tag listing page at
src/pages/blog/tag/[tag].astro with styles and static paths generation

src/lib/blog.ts

@@ -0,0 +1,17 @@
+export function formatBlogDate(date: Date) {
+    return date.toISOString().split("T")[0];
+}
+
+export function slugifyTag(tag: string) {
+    return tag
+        .toLowerCase()
+        .trim()
+        .replace(/&/g, " and ")
+        .replace(/[^a-z0-9\s-]/g, "")
+        .replace(/\s+/g, "-")
+        .replace(/-+/g, "-");
+}
+
+export function getTagHref(tag: string) {
+    return `/blog/tag/${slugifyTag(tag)}`;
+}