Add support for .env file and environment variable replacement

[yorek]

It would be great to be able to use environment variables in T-SQL Scripts, for example I could write

SELECT * FROM ${table}

and when executing the code, the ${table} will actually be replaced with the value set in the table environment variable.

Environment variables would be loaded from and .env file. If more than one .env file is present, I should be able to choose which .env file to use, and such value should be store in the .vscode folder so that I don't have to specify it every time I run the script

Ideally the support for environment variables substitution in T-SQL Script will be extended to SqlPackage.exe so that I can generate a deployment package using the .env file (or injecting environment variables using export or set)

[yorek]

Issue on the same topic: #1696