[demo] add some benchmarks

[developit]

Probably worth extracting these into a proper set of benchmarks runnable in CI using proper statistics, but I figured I'd commit some stuff I had here.

https://github.com/preactjs/signals/assets/105127/df9d318a-0234-4cc3-a381-8d21de2552e3

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Size Change: +1.65 kB (+2%)

Total Size: 82.6 kB

Filename	Size	Change
docs/dist/assets/style.********.css	1.24 kB	+30 B (+2%)
docs/dist/demos-********.js	3.44 kB	+33 B (+1%)
docs/dist/assets/bench.********.js	1.58 kB	+1.58 kB (new file)	🆕

ℹ️ View Unchanged

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