feat: new preload strategy swap-low

[SukkaW]

swap-low uses rel="alternate stylesheet" instead of rel="alternate stylesheet preload" (used by swap-high) to ensure the lowest priority.
See https://filamentgroup.github.io/loadCSS/test/new-low.html

[SukkaW]

@janicklas-ralph Kindly request your attention with a friendly ping. My PR has been open for more than a year. I would greatly appreciate it if you could spare some time to review it, at your convenience, and provide your valuable feedback.

[SukkaW]

@janicklas-ralph Kindly request your attention with a friendly ping after another three weeks. I would greatly appreciate it if you could spare some time to review it, at your convenience, and provide your valuable feedback.

[SukkaW]

@janicklas-ralph Kindly request your attention with a friendly ping after another week. I would deeply appreciate it if you could spare some time from your busy schedule to review it and provide your valuable feedback. I will resolve the merging conflict only after you have approved the change and are ready for merging.

[SukkaW]

@janicklas-ralph Kindly request your attention with a friendly ping after 26 months. I would deeply appreciate it if you could spare some time from your busy schedule to review it and provide your valuable feedback. I will resolve the merging conflict only after you have approved the change and are ready for merging.