chore(package.json): add sideEffects: false field to package.json

[TheLarkInn]

This PR adds the "sideEffects": false property in vuex's package.json file. This allow's webpack (for those who want to opt-in to requiring vuex's original source files (instead of the flattened esm bundles), then this will allow webpack to aggressively ignore and treeshake unused exports throughout the module system.

In many cases this can yield hidden and surprising build time improvements and size reductions in the case that modules flattened from rollup aren't actually needed when webpack performs a scope analysis.

The angular team has adopted this approach for the CLI yielding some strong build time speed for webpack (since unused exports are also not resolved, parsed, etc.). From my assessment of the module graph created, it looks that there are no sideEffects created against existing re-exports throughout the codebase, therefore even if a user did opt in on their own will, and remove flow types, there should be no danger of removing code that is needed and causing breakages.

Overall, this already will not be a breaking change in any way but just provides extra benefits to those (and for the cli (if realized) when using webpack 4 and mode: production). Users would still have to explicitly alias vuex (as it shows in your examples folder to realize these build speed and size wins).

Happy to answer any other questions.
~Sean + webpack team

Related: vuejs/vue#8099

[ktsn]

Thanks! Can we revert the changes under dist directory since it should be only updated on release?

[kiaking]

@TheLarkInn Can you remove the dist folder from the commit? Or I'll take over this PR. "sideEffects": false is already applied on Vuex 4, so I think it can be merged into Vuex 3 as well.