use const to replace var

[Freed-Wu]

Such as https://github.com/nodejs/node-addon-examples/blob/main/src/1-getting-started/1_hello_world/nan/hello.js

A variable declared with var is accessible in the whole module. Thus, the variable can be accessed before its initialization and outside the block where it is declared.

See MDN web docs for more details.

Unsafe fix: Use 'const' instead.

[mhdawson]

Happy to accept PRs to make those changes.

[yermartee]

Hi @mhdawson I've opened a PR for this. I've replaced var with const, and with let where mutability is required. Please review it and let me know if there are any further improvements I could make. Thanks in advance.