infra: Fix and make CI faster

[WarningImHack3r]

Something I wanted to make for a while: avoid unnecessary slow steps to be run if the modifications don't concern them.

List of changes

Performance

• Use tj-actions/changed-files to get the list of the files modified by a commit
  • Use it to conditionally skip lint checks in pre-checks (around 30s)
  • Use it to conditionally skip build, deploy & perf-check if the changes are only related to the CI (3min+)
• Use Bun for the performance-check job as it's independent and CI-only: we can win almost 30s, as tested in Migrate to Bun #114

Other changes

• Hopefully fix the +error.svelte pages not being used by reordering logic
• Remove the top-file credit, too much has changed since the file creation; it's no longer relevant
• Fix typo & remove useless id

Extensive tests & reviews have to be performed to make sure I didn't break something. Files filtering at the top of the file have to be taken into account to proofread the conditions I wrote.