feat: Add dark/light mode toggler (#4872)

**Why?** Offering light/dark mode improves accessibility and legibility
for web content. Light/dark themes are now an expectation for any modern
tooling that developers interact with.

[osv light mode
demo.webm](https://github.com/user-attachments/assets/395f22fb-1347-434a-9013-d72daa55fdf2)

**Caveats**:
- For simplicity, I wanted to keep the theming logic *strictly* on the
client side. This is why I chose localStorage instead of cookies for
storing theme state.
- I developed this with `make run-website-emulator` because I don't have
access to the real osv dataset - so perhaps the default emulator data is
not representative of the styles that would be seen in production on
real data. A thorough look against real data would be helpful to find
conflicting/unresolved styles.
- The more 'correct' way to surface light/dark mode is to include an
'auto' mode that defers to the OS-level theme setting. See
https://brandur.org/fragments/dark-mode-notes. For simplicity, I chose
to do a simple binary toggle, but it would not be very challenging to
support OS-based theming in the future.
- If you open two tabs, and flip the theme in one of them, the other tab
does not reflect the update unless you refresh. This can be fixed by
having a localStorage event listener but I chose to keep it simple.
- I did not try to pick a new colour palette, I just picked some
reasonable white/grey colours.
- I was blinded in the making of this

Author: tymzd

gcp/website/blog/hugo.toml

@@ -2,6 +2,6 @@ baseURL = 'http://osv.dev/blog/'
 languageCode = 'en-us'
 title = 'OSV Blog'
 [markup]
-[markup.highlight]
-style = 'native'
-tabWidth = 2
+  [markup.highlight]
+    noClasses = false
+    tabWidth = 2

gcp/website/blog/layouts/index.html

@@ -5,7 +5,7 @@
   <div class="author">Posted by {{ .Params.author }} on {{ .PublishDate.Format "Jan 2, 2006" }}</div>
   <div class="summary">
     {{ .Summary }}
-    <p><a href="{{ .Permalink | relURL }}">Read more...</a></p>
+    <p><a class="read-more" href="{{ .Permalink | relURL }}">Read more...</a></p>
   </div>
 </div>
 {{ end }}