obs-bilibili-stream: init at 2.1.5

Bilibili streaming plugin for OBS Studio. It adds Bilibili as a streaming
target and a dialog for managing streams, and links against obs-frontend-api.

Both build options are passed explicitly because they default to OFF in
upstream's CMakeLists.txt while upstream's own CMakePresets.json turns them on:
-DENABLE_QT=ON finds the Qt headers the dialog uses (without it the build fails
with "QMenuBar: No such file or directory"), and -DENABLE_FRONTEND_API=ON links
libobs-frontend-api, which the plugin uses to register its menu entry.

Upstream's CMake installs the plugin twice: into lib/obs-plugins/, which is the
path the OBS wrapper reads via OBS_PLUGINS_PATH, and into obs-plugins/64bit/,
OBS's own default prefix. The second copy is unused here and is removed.

Licence: the project's appstream metadata declares GPL-2.0-only. The LICENSE
file is the stock GPLv2 text and src/plugin-support.{h,c.in} are the unmodified
OBS plugin template with its placeholders unfilled, so the "or any later
version" wording there is boilerplate rather than the author's grant.

Assisted-by: DeepSeek Harness (DeepSeek-V41-Flash)
