refactor: minor improvements

[castarco]

I acknowledge that this project is in maintenance mode, but I think it could still benefit from some minor improvements:

• Use const instead of let when possible.

  • It makes the code more easy to read, plus it can help to avoid potential future bugs.
  • Although it can worsen performance for one-time executions, it does the opposite for code that executes a sufficient number of times (which is likely when working on large projects).

• Use node:* imports instead of "unqualified" imports for intrinsic modules.

  • They have been supported since Node v14.18, which is already marked as not supported.
  • These imports can help to avoid potential module naming conflicts and the associated risks.

• Mark types imports as such (available since TS v3.8)

• Use "normal" strings instead of interpolated strings when no variables are interpolated.

• Use === comparisons where it makes sense (instead of ==)

• Minor fixes on some return types