tools: fix nits in tools/doc/preprocess.js

[vsemozhetbyt]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

ES6 nits:

1. Replace remaining vars with lets.
2. Replace a common function with an arrow function in .forEach() argument.

RegExp nits:

1. Simplify [A-Za-z0-9-_] into [\w-].
2. Remove capturing and not capturing parentheses in includeExpr RegExp: there is no need to group in its case + includeExpr is used only in .match() call that does not return capturing.
3. Replace .match() with .test() in a boolean context.

Logic nits:

1. Remove excess check: .match() with g flag can only return null or non-empty array (it cannot return an array with 0 length) + incCount may be changed only after this check.

I've built the docs on Windows (using #19330) before and after these changes and both doc sets are identical + test/doctool/test-make-doc.js is OK.

[vsemozhetbyt]

CI-lite: https://ci.nodejs.org/job/node-test-pull-request-lite/277/

Let me know if we need full CI for this.

[vsemozhetbyt]

Landed in 1d42b20