Improved Target Tree generation and added extra user configurations

[sureshjoshi]

Pulled from #83 - but there were merge conflicts, and trying to use gh pr ... completely blew up locally. So, pushed the changes to a local branch, and creating a dupe PR.

Added

• Ignore Lockfiles setting so users can (un)ignore lockfiles when generating targets.
• Build Files Extension setting for users to specify an extension for BUILD files (i.e. BUILD.pants)
• A few more helper scripts in the package.json
• Helper function in configuration.ts that will return a default value for an undefined user setting (matches default value in package.json).

Changed

• Bumped pants version to latest stable 2.20.0
• configuration.ts to read new config values.
• targets-provider.ts's main getChildren() method not returns a PantsTreeItem array of both TargetTreeItems and FolderTreeItems.
• peek() now supports the Ignore Lockfiles user setting.
• vscode.open on the BUILD files now support the user-specified Build File Extension setting.