Add foldcase lookahead example

[librasteve]

From a recent thread on IRC/Discord https://discord.com/channels/538407879980482560/633753286209699870/1474676849329180682

The problem

ignorecase has some issues with Unicode comparisons

Solution provided

state the issue and suggest a more general solution with example using foldcase

[jubilatious1]

Pre-existing solutions:

~ % raku -e 'say "XßX".fc.match(/ :i ss /);'
｢ss｣

and:

~ % raku -e 'say "XßX".fc.match(/  ss /);'
｢ss｣

[librasteve]

Oh I see there’s a failing test - I will try to fix shortly.

@jubilatious1 thanks for the tips regarding match, on IRC/Discord, one aspect was how to do the fold case for just part of a string. So I’m proposing this Regex example here in the Regex section. :-)

[librasteve]

Removed trailing whitespace - hopefully this will now pass checks.

[coke]

    # Failed test 'X<Ignoremark|Adverbs,:ignoremark;Adverbs,:m> must not contain multiple indexes in one X<>'
    # at t/18-rakuast-validate.rakutest line 61
    # You failed 1 test of 738

I just went through a big PR to eliminate all the doubled X<>s - this seems to be a reversion of the change I made on this header in my PR.

[coke]

Has trailing whitespace, failing test.

[coke]

Slight preference to use single quotes here since nothing needs interpolation.

[coke]

I also edited this line in my recently merged PR, please update to match.