Add highlighting for variables

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The problem I have with the highlighting is that variables blend with the code:

puts arr

is just one color - grey, which can be confusing:

puts cr # How it looks in neovim - same color

Also variables within anonymous functions are colored just when they're defined:

10.times { |x| puts x }

only the x between the pipes is colored while the second use of x is not.
Here's a picture of a test program and the highlighting I have.

[jlcrochet]

Local variables aren't highlighted by default because some people don't like them to be; however, they are matched as crystalLocalVariableOrMethod, so you can try :hi! link crystalLocalVariableOrMethod Identifier or something similar based on your preferences.

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The command doesn't work for me.

[jlcrochet]

You're right, I forgot that crystalLocalVariableOrMethod is transparent.

[rhysd]

I think highlighting variables perfectly is not possible with highlighting system of Vim/Neovim. Since it requires Crystal parser. Until parsing the source into Crystal abstract syntax tree, it cannot be determined that the identifier is variable or not. Neovim's tree-sitter support may change the situation in the future, though.