XML Syntax highlighting should highlight template variabled with a different color

**Ricardo Bánffy** ([Bug 64401](https://bz.apache.org/bugzilla//show_bug.cgi?id=64401&redirect=false)):
When we have XML in a request body, it's displayed using syntax highlighting that doesn't differentiate between literal content and template variables (as the image shows). Ideally, the template variable should be highlighted in a different color to make it easier to see it.

Created attachment [Screenshot 2020-04-30 at 18.21.41.png](https://apache.github.io/jmeter-bugzilla-attachments/01/64401/37209/Screenshot%202020-04-30%20at%2018.21.41.png): Highlighted XML with a template variable shown as a literal

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