Add Flat LAF & IntelliJ themes support capability in JMeter

[KingRabbid]

Description

I've introduced support for Flat LAF Themes in JMeter; Flat LAF is an open source https://www.formdev.com/flatlaf/ Java Swing library with modern look & feel, supports IntelliJ themes, HiDPI, etc (see the website).
If's compatible with JMeter codebase by default.
New parameters are defined & customizable in .properties & .xml file(s): one can adjust the JSyntaxTextArea.java theme defaults.
Few additional bugs/enhancements were also introduced in order to make the visual experience better.

For future: I would remove Darcula and DarkLaf from JMeter, doesn't make sense anymore to keep them; replace DarkLaf features/usage with FlatLAF API where possible.

Motivation and Context

Due to recent eye vision problems, I've started to need alternative themes support to use other colors; existing capability in JMeter was limited.
DarkLaf API currently in-use in JMeter seems to be non maintained (last update September 2022) and complicated to upgrade to latest version.

How Has This Been Tested?

Manually tested with various JMXs & Themes, especially with JSR223 test elements to validate the JSyntaxTextArea instances.
I've run tests on both Linux and Windows, with Java 8, 11 & 17.

Note: changes are fully compatible also with JMeter 5.x branch/version if needed.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.