Fixes microsoft/vscode-docs#3724, #5561, #3295, #4761 .

Problems are:

1. The task source was incorrectly changed to "shell" (regression). Only the "type" registered task type needed to be shell: Please help me, I don't want to give up;About ADD tasks.json file vscode-docs#3724 .
2. Using Configure Default Build Task would create a task with "shell: " or "C/C++: " in the name, while F5 would not. The adding of the source to the name is caused by VS. I had added a bug report at "Tasks: Configure Default Build Task" incorrectly adds "type: " to the tasks.json "label" property vscode#98838, but I just closed it since we can work around the issue via always adding the source to the name.
3. Added the problemMatcher and group fields to the tasks created from F5 (and Build and Debug Active File): No problemMatcher added to auto-created build tasks #3295 .
4. Change the cwd to be ${workspaceFolder} instead of "/usr/bin": questionable "cwd" value in default build task #4761 . The reason we did this originally was because Cygwin would fail, but that can be worked around via either adding Cygwin's bin folder the path or changing the cwd for those cases (but most users won't want cwd to be the path to the compiler, and Cygwin usage is low compared to other scenarios).