Array and string offset access syntax with curly braces is deprecated

[lukyrys]

Array and string offset access syntax with curly braces is deprecated for php 7.4 at https://github.com/nullpunkt/lz-string-php/blob/master/src/LZCompressor/LZString.php#L59

[dducro]

Same issue.

PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in 

https://github.com/nullpunkt/lz-string-php/blob/master/src/LZCompressor/LZString.php#L22

[dducro]

PR #24 fixes this issue.

[nullpunkt]

Fixed by PR #24