Fixed typescript codegen pattern compiler

Created by: zodiia

This PR fixes an issue with the Typescript client code generator when compiling a regular expression pattern:

  Exception: Unbalanced '{' - missing '}' at 
'[A-Z]{3'
      ^

Caused by: com.github.curiousoddman.rgxgen.parsing.dflt.RgxGenParseException: Unbalanced '{' - missing '}' at 
'[A-Z]{3'
      ^

This PR supersedes #13655, as its author seems inactive. It also implements a suggestion made in this previous PR, of removing the non-standard support for leading/trailing slashes when parsing regular expressions (see https://github.com/OpenAPITools/openapi-generator/pull/13655#discussion_r992232977)

Fixes #13390 (closed)

Mentioning the Typescript technical commitee: @TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

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