KDL is a small, pleasant document language with XML-like node semantics ... to be used both as a serialization format and a configuration language, much like JSON, YAML, or XML.
all numbers can have underscores to help readability
grammar and expected semantics are well-defined and specified
before the release in addons-manager, I put lexer to git: https://github.com/CudaText-addons/CudaText-lexers
pre-alfa version is there. more coloring of the first word in the line, is it needed? some keywords need hilighting? I am sure lexer is not OK yet.
BTW, about your file 'default-config.kdl'. wiki says:
* Zip must contain: install.inf, .lcf file(s), .cuda-lexmap file per each lexer.
You can place file(s) "lexer LexerName.json" into zip archive, per each lexer in zip archive. These JSON files will be installed to the folder "settings_default" of CudaText. Ie, these are lexer-specific configs for lexers. Some lexers do this already: Lua, Pascal, PHP, Ruby, TypeScript.