Initial tasks support

[poterba]

This change addresses items #1264, #1234

In cases with file build command context, empty ws uri is possible. Additional check added.

[bobbrow]

We're still interested in taking this change. Are you planning to resubmit it?

[poterba]

Since we're providing own task type - do we need tasksBuildCommand as external command?

PS: As I've sticked to existing functionality in tasks (such as tasksBuildCommand) - I'd like to see something similar for configuration.
PPS: Why configuration isn't cancellable?

[bobbrow]

Sorry I haven't had a chance to take a look at your updates yet. I will do it soon.

If we make a change to CI, we will probably move to Azure Pipelines like the cpptools extension. This project's CI is wrapped up in a rather involved Powershell script, so we haven't come up with a cost for migrating that to another system yet.

[poterba]

UPD: misunderstanding removed

There's another part I've forgot about: buildAll.
The point is: buildAll passed to build array of ${command:cmake.buildAll} and first project path. Corresponding expand.expandString promise failed in driver.ts.

[poterba]

Ready to go (w/o test)

[julianxhokaxhiu]

Can someone please take a look at this?