Add separate guide for deps.optimizer

[sheremet-va]

Clear and concise description of the problem

There is not a lot of information about the Vitest optimizer.

Suggested solution

We should add a guide in Vitest documentation explaining how the optimizer works and where to enable it. We can rely on "Dependency Pre-Bundling" Vite docs as a source of inspiration since it uses the same mechanism.

Vitest main points:

• Importing a single file is faster than importing separate files
• CJS/ESM interop should be better
• We can use Vite plugins and aliases without processing files one by one which is faster
• Better interoperability with how Vite works in the browser

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[dammy001]

@sheremet-va I'll pick this up