Consolidate configuration setup across repository

[calebmshafer]

Update configuration to consistently use .env file throughout the repository for both test apps and integration tests. All .env files are expected to exist as a peer of the package.json.

• Deprecate use of default.json5.
• Add new custom Rush command, 'copy:config', to copy a shared .env file to each directory that requires it
  • The command does not overwrite exist files
• Enhance the internal-tools copyConfig script to allow a custom path for .env file rather than only checking a imodeljs-config directory as a peer of the imodeljs repo.
• Mark IModelJsConfig class as deprecated to be removed in 3.0.

With all of the above changes, there are two workflows for setting up,

1. Create a .env file as a peer to package.json with required variables

   Easiest for single package

2. Follow two steps to setup for all required packages
   1. Setup a .env file one of 2 ways
      • In imodeljs-config/.env or,
      • Place .env in any location and set the imjs_config_file in terminal
   2. Run rush copy:config

Note: I did plan to try and deprecate the @bentley/config-loader package in general but didn't have a great place to put the dotenv loading code. I debated moving it to the internal-tools folder, which isn't published as a package. I may do that as a follow up to this PR.

[pmconne]

Pulling to test in display-test-app.