[TS] Transpiled flatbuffers mjs directory do not contain expected modules

[bjornharrtell]

Transpilation target seems off,

Older version at https://unpkg.com/browse/flatbuffers@23.1.4/mjs/builder.js looks ok. New version at https://unpkg.com/browse/flatbuffers@23.1.21/mjs/builder.js looks like commonjs.. perhaps?

This seems to be cause of this potential problem in dependents:

ByteBuffer is not exported by node_modules/flatbuffers/mjs/flatbuffers.js

[bjornharrtell]

This regressed due to #7510.