Helper feature flags

StringKitHelper conditionally includes helper method groups. Its default is deliberately simple:

With no SK_ANY definition, SK_ALL is defined and all helper groups are available.
Define SK_ANY to opt into selective mode.
In selective mode, define one or more SK_* groups at project/build level.

fpc -dSK_ANY -dSK_CASE -dSK_ENCODE -Fusrc your_program.pas

Define	Includes
`SK_MANIP`	trimming, padding, whitespace, length, substrings
`SK_MATCH`	regex, replacement, text inspection, word extraction
`SK_COMPARE`	distances, similarities, fuzzy matching
`SK_CASE`	identifier and title conversion
`SK_VALIDATE`	common syntax validation
`SK_FORMAT`	truncation and number/file-size formatting
`SK_NUMERIC`	Roman, ordinal, number-to-words
`SK_ENCODE`	HTML, URL, Base64, hex
`SK_SPLIT`	splitting and joining
`SK_PHONETIC`	phonetics, counts, readability, n-grams

The symbols must be visible while StringKitHelper.pas is compiled. A {$DEFINE} only inside an application source file does not recompile a separately built helper unit. See Static API vs helper API and Helper coverage.