Support installing pre-release (alpha/beta/insiders) versions using VSIX

[sandy081]

Currently, extension authors get their pre-release versions tested by the community using VSIXs. But VS Code does not support installing alpha/beta/insiders versions. This force authors to always use patch for pre-release versions.

By supporting alpha/beta/insiders versions, author can generate VSIXs like pub.extname-0.11.0-beta or pub.extname-0.11.0-beta-2. At the end, they can publish the release version pub.extname-0.11.0 and all users who are using pre-release versions will get the release update.

[sandy081]

To verify:

Step 1

• Disable auto updating extensions
• Publish an extension with version 0.10.0 that has a command which will print its current version when invoked.

Step 2

• Compute a VSIX of the above extension with version 0.9.1 and prints 0.9.1 as its version
• Install it and confirm it prints 0.9.1 as its version
• Make sure the update to 0.10.0 is shown

Step 3

• Compute a VSIX of the above extension with version 0.11.0-insiders and prints 0.11.0-insiders as its version
• Install it and confirm it prints 0.11.0-insiders as its version

Step 4

• Compute a VSIX of the above extension with version 0.11.0-insiders-2 and prints 0.11.0-insiders-2 as its version
• Install it and confirm it prints 0.11.0-insiders-2 as its version

Step 5

• Publish the extension with version 0.11.0 that prints 0.11.0 as its version
• Make sure the update to 0.11.0 is shown
• Install it and confirm it prints 0.11.0 as its version

Step 6

• Try to install any older version and make sure it is installed correctly by checking the version

Step 7

• Uninstall the extension and make sure extension is not shown in the viewlet
• Quit and Start VS Code and open Extensions folder and make sure above extension folder does not exist.

[felixfbecker]

I wish the marketplace just supported release channels (e.g. latest/stable, next/unstable/insiders, ...)