allow custom rules

[cdbkr]

Is your feature request related to a problem? Please describe.
At the moment the tool supports only some pre-defined rules defined here https://github.com/IBM/tekton-lint/blob/main/src/rule-loader.ts
It would be great if users could provide (and leave them to maintain) their custom rule implementations and specify how to treat the findings (e.g. error, warning, etc..).

An example would be like the following:

const forbidPipelinesWithoutRequiredAnnotation = (docs, tekton, report) => {
    for (const pipeline of Object.values(tekton.pipelines)) {

        const pipelineAnnotations = Object.keys(pipeline.metadata.annotations);

        if (!pipelineAnnotations.includes('my/required-annotation')) {
            report(
                `Pipeline '${pipeline.metadata.name}' does not specify the 'my/required-annotation' annotation`,
            );
        }
    }
};

module.exports = {
    rules: {
        'no-required-annotations': 'error'
    },
    customRules: {
        'no-required-annotations': forbidPipelinesWithoutRequiredAnnotation
    }
}

Describe the solution you'd like
Support (on top of the yaml configuration) a JS (or TS) configuration file as well, e.g. .tektonlintrc.js where it is possible to define a set of custom rules or provide a specific rule as a function (instead of providing a string 'error' or 'warning', and leave the implementation to handle it).

Describe alternatives you've considered
At the moment there is no way to provide custom rules since they are specified in the mentioned file above

Additional context
Add JS File loader in the ./src/config.ts main constructor. An example below:

if (fs.lstatSync(user_tektonlintrc).isDirectory()) {
    user_tektonlintrc = path.join(user_tektonlintrc, '.tektonlintrc.yaml');
}

if (fs.lstatSync(user_tektonlintjs).isDirectory()) {
    user_tektonlintjs = path.join(user_tektonlintjs, '.tektonlintrc.js');
}

let customRcConfig = {
    ...defaultConfig
};

if (fs.existsSync(user_tektonlintrc)) {
    const customRcFile = fs.readFileSync(user_tektonlintrc, 'utf8');
    customRcConfig = yaml.parse(customRcFile);

    logger.info('Using .tektonlintrc.yaml at %s', user_tektonlintrc);
    logger.info('customConfig %o', customRcConfig);
} else if (fs.existsSync(user_tektonlintjs)) {
    customRcConfig = require(user_tektonlintjs);

    logger.info('Using .tektonlintrc.js at %s', user_tektonlintrc);
    logger.info('customConfig %o', customRcConfig);
} else {
    logger.warn('Unable to find configuration - using defaults');
}

this._rulesConfig = { ...customRcConfig };
this._rulesConfig.rules = { ...customRcConfig.rules };
this._rulesConfig.customRules = { ...customRcConfig.customRules };
this._globs = [...cliConfig['_'], ...(customRcConfig.globs ? customRcConfig.globs : ['_'])];

[mbwhite]

Work is underway in PR #106

[mbwhite]

I've merged the PR, and updated the documentation; there is a sample custom rule in the repo as well.

The implementation is very similar to that originally suggested.