[v2] accidentally create a global variable `window.value`

[drify]

https://github.com/mathjax/MathJax/blob/legacy-v2/unpacked/extensions/toMathML.js#L71

The above line accidentally create a global variable value, which may cause problems if other codes rely on window.value
This issue affects all sites with MathJax v2, including Stack Exchange: open this page and window.hasOwnProperty('value') will be true

Add the missing var will fix this issue.

[dpvc]

Thanks for catching that. I'll make a PR for it.