Use modern JavaScript syntax for code examples

[atombrenner]

The goal of this PR is to use common modern JavaScript syntax in the code examples of the documentation. This will increase the readability of the examples by being more familiar syntax. I should also help spread best practices with modern, as people tend to copy paste examples.

There are only two types of changes:

1. Using const instead of var I found no examples that need a let. Sometimes a code example declared the same variable multiple times, usually to explain overloads. In this case I splitted the example.
2. Using arrow functions instead of anonymous functions This mainly improves readability by reducing some noise. Formatting is by Prettier conventions.

While doing those changes I stumbled over two potential bugs, please review the commits

• https://github.com/Yomguithereal/mnemonist/commit/a6d02110229a8a8b843a1cf66b5e134350e7cbc0 syntax in FuzzyMultiMap.from() example was wrong, fixed to the best of my knowledge
• https://github.com/Yomguithereal/mnemonist/commit/e0a35d27ac0acdf4fd0e7ab2f5e5d92e4d43bdd9 same problem as before in search-index but this file does not exist (maybe moved to inverted-index?)

[Yomguithereal]

Thanks @atombrenner, I will review your PR shortly. Regarding the search-index I think you can safely drop the file since the structure was indeed renamed to InvertedIndex. The inverted-index also has a typo where I wrote Index instead of InvertedIndex.

[Yomguithereal]

Thanks @atombrenner. Will merge now.