Add icon to external links

[LeMurphant]

fixes #863

There were a few modifications compared to what was done in #346, which Claude says give better compatibility, but @Aprillion LMK if it's wrong

[LeMurphant]

Unfortunately, I have noticed that in rare cases, the icon can appear after the line-wrap. I was not able to fix this.

[Aprillion]

Unfortunately, I have noticed that in rare cases, the icon can appear after the line-wrap. I was not able to fix this.

I pushed a commit with position:absolute fix/workaround, feel free to test on https://stampy-ui2.aprillion.workers.dev/questions/6953/Do-people-seriously-worry-about-existential-risk-from-AI and revert if not good for all edge cases.

I also fixed line height of lines with footnotes using <sub> to stay the same as lines without footnotes, because this workaround depends on that:

[LeMurphant]

@Aprillion your fix works on Firefox (it did not work before), but on Chrome I still get the same result as before.

[Aprillion]

hmm, previous version looked good in Safari too ... maybe Chrome optimized performance of some more reasonable CSS and not for absolutely positioned pseudoelements inside inline line-wrappable parents 🙈

now using a different workaround, to bake in the background directly to the element without :after - but now we can only choose 1 color, I chose the visited link color because it looked better for both unvisited and visited links (while the unvisited color looked strange for visited links) ... feel free to choose another fixed color if this workaround wins

[LeMurphant]

@Aprillion this looks good to me, do you mind re-approving the PR so I can merge?