I’m not aware of a prebuilt version, but you can build a tool called basisu from here: GitHub - BinomialLLC/basis_universal: Basis Universal GPU Texture Codec.
That said, you might want to try .ktx2 instead of .basis. The software is here: GitHub - KhronosGroup/KTX-Software: KTX (Khronos Texture) Library and Tools with prebuilt binaries in releases. You can use toktx to compress images to BasisU and load them in Babylon. Note that this code path is still experimental, but it should be fairly stable.