Add commands to get preset names

[benmcmorran]

This changes commands for command substitution and associated documentation.

The following changes are proposed:

3 new commands: cmake.activeConfigurePresetName, cmake.activeBuildPresetName, cmake.activeTestPresetName. Each returns a string with the name of the corresponding preset, or an empty string if the preset is not available.

The purpose of this change

These commands are designed to be used with the command substitution syntax in launch.json or tasks.json. They help to write generic launch configurations or tasks in the presence of CMake projects with multiple presets that may use the ${presetName} macro to select the binaryDir. While the existing cmake.launchTargetPath can often also be used in this situation, it can't handle constructing the path to files that aren't the target (e.g. associated debug information or, for embedded applications, hex or bin files).

FYI @aleun @gcampbell-msft

[elahehrashedi]

@benmcmorran there has been some renaming recently, can you apply it to your branch so that the builds pass, they are failing currently.