Extension terminal can accidentally be spawned twice

[andyleejordan]

Prerequisites

• I have written a descriptive issue title.
• I have searched all open and closed issues to ensure it has not already been reported.
• I have read the troubleshooting guide.
• I am sure this issue is with the extension itself and does not reproduce in a standalone PowerShell instance.
• I have verified that I am using the latest version of Visual Studio Code and the PowerShell extension.
• If this is a security issue, I have read the security issue reporting guidance.

Summary

As reported on the Discord by @PrzemyslawKlys, it is possible to spawn the extension terminal twice on accident. If the extension is in the process of starting, and another event initiates a start, two terminals will be spawned. The start() function of the session manager is not re-entrant, but as there are multiple events that may enter it, it needs a simple lock that waits for the original call to finish if it's called while already starting.

PowerShell Version

N/A

Visual Studio Code Version

N/A

Extension Version

v2022.8.5

Steps to Reproduce

While extension is still starting, do something like run a PowerShell debug config which also starts the extension.

Visuals

No response

Logs

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