[IDE] Slow Startup - Create Examples menus for Libraries (need ignore files)

Hi, I realized that also the code that builds the examples of the libraries, may have a problem. If the library has large documentation (docs) like generated by doxygen, or some other directory with many files, the IDE will take a long time to scan.

My first idea was to ignore the known files ... as is done with .svn and .git ... but I ended up identifying that it is better to do this treatment below:

https://github.com/arduino/Arduino/blob/02a2e22649feb4d85d807b51122f2501b06da866/app/src/processing/app/Base.java#L1779-L1781

to this:
To only focus on the folder of interest.
```
private boolean addSketchesSubmenu(JMenu menu, UserLibrary lib) {
File examples = new File(lib.getInstalledFolder(), "examples");

JMenu submenu = new JMenu(lib.getName());
boolean found = addSketches(submenu, examples);
if (found) {
menu.add(submenu);
MenuScroller.setScrollerFor(submenu);
}

return found;
}
```
In my case I had a node_modules folder (don't ask me why ... lol) it took about 5s to read it
total time was around 11s, because i had another library with a big docs folder.

PS: Sorry for opening several separate problems, but it is for us to track the errors I found. As it is part of an ongoing refactoring, in order to solve part of the same "slow startup" problem, it is difficult to make several separate pull requests.

Read app/src/processing/app/Base.java around the linked addSketchesSubmenu code and trace how library examples are scanned. Reproduce startup with a library containing a large docs or node_modules directory; done means example-menu creation focuses on the examples folder and avoids scanning unrelated files.

Written by the indexing model from the issue text.