lib: use const where possible

[JungMinu]

use const instead of var where possible

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[nodejs-github-bot]

@JungMinu build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/2920/pipeline

[Trott]

We usually reject PRs for the lib directory that are exclusively var -> const like this one. A recent previous example is #26504. See the conversation there for some of the reasoning.

[Trott]

If you want to try to get a var -> const PR for the lib directory accepted, I'd recommend this approach:

• Configure ESLint to flag instances of var that could be const. (Don't flag instances that would have to be let.)

• Auto-fix the entire lib directory.

• Submit the changed lib code along with the ESLint configuration change.

The PR may still get rejected due to churn etc.! But there is at least some support for the idea that we keep getting these kinds of PRs so it may be time to just fix it once and for all. So it might get accepted.

[BridgeAR]

Superseded by #26915