Add performance testing

[riccardoferretti]

We need to have some performance tests for Foam.

Approaches and notes:

• generate a directory with various files connected based on some patterns (random, with some sort of linear progression, ...).
• use existing digital gardens (see https://github.com/KasperZutterman/Second-Brain and https://github.com/foambubble/foam/pull/467#discussion_r568144587)
• test also performance with large md files (see https://github.com/foambubble/foam/issues/466#issuecomment-771589901)

Anything else?

[leonhfr]

Batch operations:

• renaming a directory
• N files deleted at once
• N files added at once (moved from outside foam)