package.json: specify sideEffects: false

Minimize data files, remove unnecessary files.

• Improve performance of encode(), decode() and decodeEntity() by using function inlining.
• Fix decoding HEX HTML entities in some cases.

Add flow types.

A fast decodeEntity() method to decode a single HTML entity.

• Speed up both encode() and decode() methods.

• Add extensive mode to encode() method. This mode encodes all non-printable characters, non-ASCII characters and all characters with named references.

• Handle invalid numeric HTML entities: mimic browser behaviour.

• Handling behaviour of ambiguous ampersands.

• Fix webpack build warning.