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.