Add transformTagName option to transform tag names when parsing

[erkie]

Purpose / Goal

Add an option to transform tag names in output of .parse. Option name is:

transformTagName: false | (tagName: string) => string;

Pasting the test here because it's a great sample of what this PR does:

const xmlData = `<?xml version='1.0'?>
  <root>
    <person>Person 1</person>
    <Person>Person 2</Person>
    <PERSON>Person 3</PERSON>
    <person>Person 4</person>
  </root>
`;

const parser = new XMLParser({
    ignoreDeclaration: true,
    transformTagName: (tagName) => tagName.toLowerCase()
});
let result = parser.parse(xmlData);

const expected = {
    "root": {
        "person": [
            "Person 1",
            "Person 2",
            "Person 3",
            "Person 4"
        ]
    }
};

expect(result).toEqual(expected);```

# Type
Please mention the type of PR
<!-- choose one by changing [ ] to [x] -->
* [ ]Bug Fix
* [ ]Refactoring / Technology upgrade
* [x]New Feature

[amitguptagwl]

Thanks for this PR @erkie . I'm just wondering that there can be multiple options to format the tag or attribute name. And probably everyone need some combination as per their need. So it could be better to create a function that can be overridden by the user to apply particular formatting.

In this way, some more requirements can be covered eg replacing hyphen with camel casing etc.

[erkie]

Thanks for the feedback @amitguptagwl I totally agree with you. I amended the PR to be transformTagNames instead so it's 100% customizable now.

[amitguptagwl]

It looks like some tests are failing. Can you please check?

 Failures:
1) XMLParser should parse lowercase tagnames
  Message:
    Expected object not to have properties
        ?xml: ''
  Stack:
    Error: Expected object not to have properties
        ?xml: ''
        at <Jasmine>
        at UserContext.<anonymous> (/home/runner/work/fast-xml-parser/fast-xml-parser/spec/transform_tagname_spec.js:9:374)
        at <Jasmine>

[erkie]

@amitguptagwl thanks for that, it was an error with my tests. It has been fixed now.

Also, I changed the option name from transformTagNames to transformTagName since it made more sense.

[coveralls]

Coverage decreased (-1.3%) to 98.544% when pulling 5e38a80 on feederco:lowercase-tag-names into 2203711 on NaturalIntelligence:master.

[coveralls]

Coverage decreased (-1.3%) to 98.544% when pulling 5e38a80 on feederco:lowercase-tag-names into 2203711 on NaturalIntelligence:master.