Enhanced vi-like word motions in copy mode

[eduardorittner]

Item 2 from #4471, which are more complete vi motions. Before the PR, copy-mode already had:

• w for moving to the next word
• e for moving to the end of WORD (not word)
• b for moving to the start of word

And now it has

• W for moving to the next WORD
• e for moving to the end of word (fixed)
• E for moving to the end of WORD
• B for moving to the start of WORD

In the code, word and WORD are called word and non_blank_word, respectively, since vim's definition of WORD is a sequence of non-blank characters separated by whitespace. These bindings were manually tested and seem to work as intended, however I thought of adding tests to validate the behavior but found no examples of how to do so, any tips would be greatly appreciated.