Consider moving the bundled copy of HarfBuzz to a distinct optional crate

[hsivonen]

Currently, harfbuzz-sys contains 1) a copy of HarfBuzz itself and 2) a build.rs that probes for system HarfBuzz.

The bundled copy of HarfBuzz poses the problem that when a project with code browsing tools does not use the bundled copy of HarfBuzz, the bundled copy is confusing for code search and code browsing tools, and potentially for SBOM analysis.

The build.rs was a blocker for me to get the crates building for Android.

For projects that already manage the build of HarfBuzz itself in other ways, these issues would be solved by moving the bundled copy of HarfBuzz and the associated build.rs into an optional distinct dependency of harfbuzz-sys. Something like harfbuzz-bundled that woud be off by default and could be included by bundled feature of harfbuzz-sys.