Add script to find slow-to-highlight files

[sharkdp]

This PR adds a script that goes through all languages that are supported by 'bat'. For each language, it loops over the correspoinding file extensions and searches a given folder for matching files. It calls 'bat' for each of these files and measures the highlighting speed (number of characters per second). The script reports files which lead to slow highlighting speeds or errors during the execution of 'bat'.

Here are some results:

• I found a couple of C# *.cs files. This was expected and will be fixed in the next release (see slow output, with pauses, with c# source code #677)

• I found a couple of *.gemspec and *.rake files (Ruby) which lead to a panic. This is a minified example:

  <<END_DESC
  Test
  END_DESC

  Apparently, there is a problem with heredocs in the Ruby syntax: https://github.com/sublimehq/Packages/blob/f36b8f807d5f30d2b8ef639232a9fc5960f550fa/Ruby/Ruby.sublime-syntax#L4-L17
  (note how GitHub also highlights this incorrectly)

  Nevertheless, we obviously shouldn't panic here:

  thread 'main' panicked at 'regex string should be pre-tested: Error(-208, invalid backref number/name)', /home/shark/.cargo/registry/src/github.com-1ecc6299db9ec823/syntect-4.1.0/src/parsing/regex.rs:70:17
  

  For some reason, this was not caught when creating the syntax binary dump where the regex should have been pretested. The offending regex is ^\1$. Bug ticket: bat panics on Ruby files with unindented heredocs #914

• I found a *.vue file that leads to a panic:

  thread 'main' panicked at 'Can only call resolve on linked references: ByScope { scope: <source.stylus>, sub_context: None }', /home/shark/.cargo/registry/src/github.com-1ecc6299db9ec823/syntect-4.1.0/src/parsing/syntax_definition.rs:188:18
  

  The minified example looks like this:

  <style lang="stylus">
  </style>

  Bug ticket: bat panics on simple Vue file #915

• I found a couple of files which are highlighted rather slowly, but nothing as drastic as the Makefile (Slow output with some Makefiles #750) and C# (slow output, with pauses, with c# source code #677) issues