feat(core): ldml keyboard transform elements

Adds definitions for LDML keyboard transform elements `transforms`,
`backspaces`, and `reorders`, and the binary format sections `bksp`,
`elem`, `finl`, `ordr`, and `tran`.

Minimally updates the compiler so that it will build, but no other
changes to the compiler.

Compiler and Typescript updates to support building these sections will
come in the next PR.