* .gitlab-ci.yml: Initial introduction of CI.

This configuration only supports Meson build on Linux. Windows and macOS would
be supported in the future. There's no plan to support autotools and CMake by
now.

The Linux build leverages `ci-templates` project to build the base image for
the build environment.
2 jobs for ci in 1 minute and 16 seconds (queued for 1 second)
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