Stop scanning project file when a maximum limit is reached

To avoid excessive memory usage when opening in a directory with too many files.

Introduce the config variable config.max_project_files to choose the limit.

The mechanism introduced avoid using excessive memory but it fails to
let user access all the files in the directory. A better implementation
should not impose any limits but read each subdirectory on-demand, only
as they are expanded in the tree-view.

data/core/config.lua

@@ -17,5 +17,6 @@ config.indent_size = 2
 config.tab_type = "soft"
 config.line_limit = 80
 config.max_symbols = 2000
+config.max_project_files = 2000
 
 return config