Vertical whitespace between functions

[bunnyhero]

As requested in #2526, adds a configuration option to vertical_whitespace rule that allows specifying a separate value for the maximum number of empty lines between functions.

Notes:

• The new option is named max_empty_lines_between_functions. If the option is absent, then its value defaults to the value of max_empty_lines. I was uncertain of the best way to handle this, and if there's a simple way to note how this works in the generated documentation.
• I use a linear search to detect whether a section of empty lines is adjacent to a function. Obviously this isn't very efficient. I haven't benchmarked this however.

[SwiftLintBot]

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