tools: make some changes to .editorconfig

1. The src directory does not contain any JavaScript source files, so there is no need to include the directory in the rule definition.
2. The tools directory contains JavaScript files which should be matched, most importantly tools/eslint-rules and tools/doc.

One side-effect of the second change is that the following rules now apply to some other files in the tools directory which were previously ignored:

end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true

Will this cause problems for anyone?

Checklist

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