Display coverage results from `bazel coverage`

VS Code added support for code coverage recently (See https://github.com/microsoft/vscode/issues/123713). It probably makes sense to integrate this with `bazel coverage`, such that the code coverage is directly shown in the IDE

* [x] Integration with coverage tasks defined in `tasks.json` #366
* [x] LCOV parsing
* [x] Function coverage #366
* [x] Line coverage #366
* [x] Branch coverage #389
* [x] name demangling
* [x] Java #388
* [x] C++ https://github.com/bazelbuild/vscode-bazel/pull/398
* [x] Rust https://github.com/bazelbuild/vscode-bazel/pull/398
* [ ] Setting to display only boolean coverage instead of hit-counts
* [ ] Builtin commands and integration into "Bazel Build Targets" treeview

Start by reviewing the existing coverage-task integration in tasks.json and the current Bazel Build Targets treeview. Compare the remaining checklist items—boolean-only coverage display, builtin commands, and treeview integration—with what is already implemented; done means these items work together with the existing LCOV coverage support.

Written by the indexing model from the issue text.