add initial support for xcompose conversion to yamlcompose

[Granitosaurus]

Linux's xcompose mappings support .xcompose to .yaml conversion:

$ gen-compose-conver xcompose --help
Usage: gen-compose-convert xcompose [OPTIONS] [FILES]...
  Convert xcompose file, that follows format like: <Multi_key> <parenleft>
  <period> <1> <parenright>: "⑴"
Options:
  -c, --keep-comments  keep inline comments
  --help               Show this message and exit.

For example:

$ cat mappings/example.compose
<Multi_key> <B> <bar> : "₿" U20BF  # BITCOIN SIGN
$ gen-compose-convert xcompose mappings/example.compose --keep-comments > example.yaml
$ cat example.yaml
"B|": "₿"  # BITCOIN SIGN