feat(nested config files)

Adds a config_files option to config.json that allows the user
to specify one or more config files to load in addition to the
main config file.

For me this allows me to keep my flow configuration in a shared dotfiles
repository managed by git.

src/config.zig

@@ -22,3 +22,5 @@ top_bar: []const u8 = "",
 bottom_bar: []const u8 = "mode file log selection diagnostics keybind linenumber clock spacer",
 
 lsp_request_timeout: usize = 10,
+
+config_files: []const u8 = "",