support "_" within numbers

[lcmgh]

When using large numbers once can write 5_000 instead of 5000 in Rust. Using such a format in config files does not work.

[epage]

Some config file formats support this natively, like toml.

For us to support it in those that don't would require treating the value as a string. For formats with bare words, like yaml, that would be transparent (5_000). For others, it wouldn't ("5_000").