Verymagic (#190)

* fix substitution with empty string

* fix input dialog overlapping key info

* support backreferences in replacement pattern

* improve vim regex support

* better explain reason why when regex doesn't match

* Update src/vim.js

Co-authored-by: Faris Masad <farismasad@gmail.com>

* Update src/vim.js

Co-authored-by: Faris Masad <farismasad@gmail.com>

* fix tests

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Co-authored-by: Faris Masad <farismasad@gmail.com>