Solution for https://github.com/Huachao/vscode-restclient/issues/858

[BlackJohnny]

Ref: #858

Replaced the default variable processing by replacing JSONwith JSONbig for both parse and stringify.

The changes to webpack.config.js are to avoid an issue with the usage of bignumber.js by the json-bigint package under webpack.

[BlackJohnny]

Until this is accepted, a build containing latest code changes to date of this comment can be found here: https://github.com/BlackJohnny/vscode-restclient/releases/tag/v0.24.5