Allow for loop increment expression to use variable defined inside loop

[sirbrillig]

In most cases, for loops do not require special processing by this sniff because they share the variable scope of their container. However, the third expression of the loop (what I'm calling the "increment expression") is technically not evaluated until after the loop runs at least once. This means that you could define a variable inside the loop that is then used in the increment expression, despite the definition coming lexically after the use.

In this PR, we modify the sniff to set aside variables in the increment expression until after the loop has been parsed. Variables found in this way are then parsed in their own operation (but in their original position).

Fixes #261

[jrfnl]

Little heads-up for when you continue working on this: each "expression" segment in a for can contain multiple comma-separated expressions.
Example: https://3v4l.org/X3R8U

[sirbrillig]

Ugh, trying to find the semicolons that divide a for loop's initialization, condition, and increment expressions is really hard since each expression could contain a closure that itself contains semicolons. 😩

[jrfnl]

Ugh, trying to find the semicolons that divide a for loop's initialization, condition, and increment expressions is really hard since each expression could contain a closure that itself contains semicolons. 😩

Haven't looked at your current code, but in case you aren't already, you may want to use something along the lines of this code to jump over closed structures: https://github.com/PHPCSStandards/PHPCSUtils/blob/b65fbd47c38202a667ea3930a4a51c5c8b9ca434/PHPCSUtils/Utils/PassedParameters.php#L243-L280 (That's the code I use to loop through and split out the parameters in a function call, where parameter values may contain the comma delimiter as well)

[jrfnl]

You may also find this code useful to see: https://github.com/PHPCSStandards/PHPCSUtils/blob/b65fbd47c38202a667ea3930a4a51c5c8b9ca434/PHPCSUtils/Utils/Context.php#L152-L204 (utility which splits a for() into the three expressions)

[sirbrillig]

Thank you, @jrfnl! That was the key; I needed to check for the level of the tokens I was scanning.