wt new-tab open near current tab

# Description of the new feature/enhancement

I have a Bash function that opens a tab with PowerShell in the current directory:

```sh
vspwsh ()
{
wt new-tab --profile "Developer PowerShell for VS 2017" --startingDirectory .
}
```

I'd like this to place the new PowerShell tab next to the Bash tab in which I run the command. That way, it would be easier for me to keep track of the purpose of each tab, keeping tabs near each other when they are for working on the same directory.

This is somewhat related to but would not depend on any escape sequences emitted by the shell.

# Proposed technical implementation details (optional)

Add a `--near-current-tab` option to `wt new-tab`. When specified, use the WT_SESSION environment variable to find the session, tab, and window in which the command was run, and place the new tab near that one.

If both `--near-current-tab` and `--window` are specified, then ignore WT_SESSION and instead use the selected tab of the specified window.

Start at the `wt new-tab` command entry point and trace how `WT_SESSION` identifies the current session, tab, and window. Check how `--window` selects a window, then define the behavior for `--near-current-tab`, including when both options are supplied; done means the new tab opens next to the invoking or selected tab.

Written by the indexing model from the issue text.