remove compatibility ft_ functions that are not used anywhere

remove compatibility idx_ functions that are not used anywhere

make public listIndex() and listIndexLengths() method

because used in inc/infoutils.php file

use Indexer method instead of idx_get_indexer()

new namespace dokuwiki\Search

deprecated class Doku_Indexer extends \Indexer

coding style PSR-12

encapsulate functions into Classes

class FulltextSearch
class MetaSearch

make Doku_Indexer singleton

encapsulate wordlen() in the Indexer

encapsulate idx_get_stopwords()

encapsulate idx_get_version()

encapsulate idx_addPage()

Note: idx_listIndexLengths() is used in iinc/TaskRunner.php and inc/Remote/ApiCore.php file

Also used in _test files.

deprecated idx_lookup()

This function is not called from elsewhere.

deprecated idx_tokenizer()

This function is not called from elsewhere.

deprecated idx_getIndex()

Note: idx_getIndex() is used in inc/infoutils.php file

encapsulate idx_listIndexLengths()

Note: idx_listIndexLengths() is used in inc/infoutils.php file

deprecated idx_indexLengths()

This function is not called from elsewhere.

encapsulate idx_cleanName()

idx_cleanName() was called only from Doku_Indexer::addMetaKeys(), lookupKey(), getPages(), histogram()

coding style

PHP8 fix: string offset out of range due to null

PHP8 fix part 5: Trying to access array offset on value of type bool/null

PHP8 fix part 4: Trying to access array offset on value of type bool/null

PHP8 fix part 3: Trying to access array offset on value of type bool/null

PHP8 fix part 2: Trying to access array offset on value of type bool/null