spell: Enable spell checking in strings

[MatejKastak]

This PR enables spell checking within strings.

Most of the other syntax specifications enable this by default (even built in ones):

• https://github.com/neovim/neovim/blob/c1fbc2ddf15b2f44b615f90b2511349ab974cb83/runtime/syntax/c.vim#L60
• https://github.com/neovim/neovim/blob/c1fbc2ddf15b2f44b615f90b2511349ab974cb83/runtime/syntax/java.vim#L182
• https://github.com/neovim/neovim/blob/c1fbc2ddf15b2f44b615f90b2511349ab974cb83/runtime/syntax/python.vim#L139
• https://github.com/vim/vim/blob/master/runtime/syntax/python.vim (even the original vim)

[udalov]

Thanks!