Fixed isJansiConsoleInstalled performance issue

[ChrisTrenkamp]

Fixes #1975

isJansiConsoleInstalled() was repeatedly called
in a loop from the TextTable#toString method,
which caused performance problems due to the
constant calls to Class#forName. This change
caches the result from isJansiConsoleInstalled so
it doesn't repeatedly call Class#forName, which
cannot change at runtime.

[remkop]

@ChrisTrenkamp I had to enable CI/CD for this PR, but now I see some tests are failing:

## On Java 7 Ubuntu
picocli.HelpAnsiHeuristicsTest > testAnsiAutoJansiConsoleInstalledOverridesHintDisabled FAILED
    java.lang.AssertionError at HelpAnsiHeuristicsTest.java:373

and

## On Java 18 Windows
picocli.HelpAnsiTest > testAnsiEnabled FAILED
    java.lang.AssertionError at HelpAnsiTest.java:132

This may be due to the order in which tests are executed.
Perhaps all JansiConsoleInstalled-related tests need to be moved to separate test classes (a separate class per test)?

[ChrisTrenkamp]

The failing test cases are passing now, but it had to be done by resetting the cached value. Doesn't feel like the most elegant way to handle this.

[remkop]

I see what you mean but I cannot think of an alternative either. 😅
I think this is fine.
Thank you fixing the tests!

[remkop]

Having some trouble with the CI/CD, but I do see this failing test:

picocli.HelpAnsiHeuristicsTest > testAnsiAutoJansiConsoleInstalledOverridesHintDisabled FAILED
    java.lang.AssertionError at HelpAnsiHeuristicsTest.java:378

[remkop]

Merged. Sorry, it took some time to sort out the CI/CD issues.
Many thanks for the contribution!