This applies https://github.com/troglobit/editline/pull/70 to our build
of editline, which translates `meta-left` and `meta-right` into
`fd_word` and `bk_word`. This makes `nix repl` soooo much nicer to use!
Note: My terminal renders `meta-left` as `\e\e[C` and `meta-right` as
`\e\e[D`.
Closes https://git.lix.systems/lix-project/lix/issues/501
Change-Id: I048b10cf17231bbf4e6bf38e1d1d8572cedaa194