Add memory utilization to benchmark

[Balearica]

Tesseract.js has had multiple performance and memory-related bugs, which unfortunately are not flagged by automated testing. For performance-related bugs this was resolved by adding a benchmark in #628. We should expand this to also report memory usage so excessive memory use can be identified.

[Balearica]

I added code to the browser speed benchmark to report memory usage both (1) after initializing the worker and (2) after running recognition. Notably, this will not tell us the peak memory usage during recognition, however it should flag any memory leaks or excessive memory use between jobs.

The number reported on my system (Chrome + Ubuntu) were 303.0 MB after initialization and 312.0 MB after recognition. This only includes the memory used in the single worker (not e.g. the main thread or the DOM). This seems high as it means just 3 worker will push memory usage >1GB during recognition, so hopefully we can figure out how to reduce for Tesseract.js v5.

The new(ish) function performance.measureUserAgentSpecificMemory is used to report memory usage. This allows for reporting the memory of both the main thread and workers, which unfortunately was not possible with performance.memory. Unfortunately, this requires specific server settings to work for security reasons, so it's possible somebody will encounter issues with different server configurations. I've edited the included server.js script so using npm run should work.