Add kdl.vim to support the KDL document/config language

[openjck]

What is the name of this language in Linguist?

KDL is not listed in Linguist.

Link to GitHub repository of Vim plugin

https://github.com/imsnif/kdl.vim

Is this plugin well maintained?

The plugin has seen relatively little activity since its creation in September, 2022.

Is this plugin lightweight? (no advanced functionality, just indent and syntax support)

Yes, the plugin appears to be lightweight.