Supports a relative journal_file

[SharzyL]

Currently journal_file option is parsed with lsp_types::Url::from_file_path

beancount-language-server/crates/lsp/src/server.rs

Line 122 in 4332edd

let journal_root = lsp_types::Url::from_file_path(file)

Since from_file_path only accepts absolute path, the lsp server will panic with a relative journal_file. It would be convenient and portable if the lsp server can accept a relative path, e.g. ./root.beancount.