Change activation of conda environments in powershell

[DonJayamanne]

Once the upstream Conda Issue #626 is resolved, we'll need to update the conda activation accordingly.

[kalefranz]

A contributed powershell wrapper to conda would be hugely appreciated. With conda 4.4+, there are five other shell wrappers for conda. We're still obviously missing powershell.

I started the conda/conda#6471 PR as a sketch of what the powershell wrapper might look like, and Steve Dower added some additional comments in conda/conda#6471 (comment).

Eager to work with anyone wanting to push this through and get it in.

[DonJayamanne]

Prescribed solution:

• Modify https://github.com/Microsoft/vscode-python/blob/ae1848a82f5af8983f663d5d4db2c4f77ad62c57/src/client/common/terminal/environmentActivationProviders/condaActivationProvider.ts
  Currently the block of code is empty for powershell, see below:

    public async getPowershellCommands(
        _envName: string,
        _targetShell: TerminalShellType
    ): Promise<string[] | undefined> {
        return;
    }

• Modify to send the command conda activate xxx
• Assumption - Powershell has been configured on user environment for conda.

[tonyxwz]

My current solution is to add the following to workspace setting,

{
"terminal.integrated.shellArgs.windows": [
        "-NoExit",
        "-Command",
        "conda activate pyHIFU"
    ],
    "python.terminal.activateEnvironment": false
}

where pyHIFU is my environment name.

[karrtikr]

Given powershell has been configured on user environment for conda and conda activate <envName> works on powershell, validated.