codegen: add `<>` -> 'Not_Equal' to specialCharReplacements

Created by: Felk

<> is a fairly common alternative to !=, e.g. in SQL or in DevExtreme filters. Previously, if one of an enum's variant was just '<>', its entire name would be sanitized away, resulting in an empty string as symbol name and therefore a syntax error.

---

I have an OpenAPI schema that looks something like this:

{
  // ...
  "components" : {
    "schemas" : {
      "FilterOp" : {
        "enum" : [ "=", "<>", ">", ">=", "<", "<=", "contains", "notcontains", "startswith", "endswith" ],
        "type" : "string"
      },
      // ...
    }
  }
}

The OpenAPI-Typescript-Generator currently emits the following code:

/**
 * 
 * @export
 * @enum {string}
 */
export enum FilterOp {
    Equal = '=',
     = '<>',
    GreaterThan = '>',
    GreaterThanOrEqualTo = '>=',
    LessThan = '<',
    LessThanOrEqualTo = '<=',
    Contains = 'contains',
    Notcontains = 'notcontains',
    Startswith = 'startswith',
    Endswith = 'endswith'
}

Which has an empty string for the symbol name of the '<>' enum variant and therefore does not compile. This happens because AbstractTypeScriptClientCodegen#toEnumVarName sanitizes invalid characters using sanitizeName, and <> consists of such invalid characters only, resulting in an empty string.

For a fixed set of strings defined in specialCharReplacements this is solved by providing replacement. I opted for adding a replacement for <> there. With that change the following code is emitted instead (Not_Equal gets sanitized to NotEqual):

/**
 * 
 * @export
 * @enum {string}
 */
export enum FilterOp {
    Equal = '=',
    NotEqual = '<>',
    GreaterThan = '>',
    GreaterThanOrEqualTo = '>=',
    LessThan = '<',
    LessThanOrEqualTo = '<=',
    Contains = 'contains',
    Notcontains = 'notcontains',
    Startswith = 'startswith',
    Endswith = 'endswith'
}

This merge request solely fixes a specific usecase I have. It does not fix this issue in general, which is that sanitizeName in DefaultCodegen may always result in an empty string for some inputs. I do not know how that would be best addressed.