Add 'tasks.registerTaskProvider' for Plugin API

[RomanNikitenko]

Add 'tasks.registerTaskProvider' for Plugin API
Task Provider allows a plugin to provide and resolve tasks.

Example:

theia.tasks.registerTaskProvider('shell', {

    provideTasks: token => {
        const buildTask = {
            name: 'yarn build',
            definition: {
                type: 'shell'
            },
            execution: {
                command: 'yarn',
                args: ['run', 'build'],
                options: {
                    cwd: '/home/projects/task-provider-plugin'
                }
            },

        };

        const watchTask = {
            name: 'yarn watch',
            definition: {
                type: 'shell'
            },
        }

        tasks.push(buildTask);
        tasks.push(watchTask);

        return tasks;
    },

    resolveTask: task => {
        if (task.name === 'yarn watch') {
            task.execution = {
                commandLine: 'yarn run watch',
                options: {
                    cwd: '/home/projects/task-provider-plugin'
                }
            }
        }
        return task;
    }
});

`
Build task in the example has name, definition and execution sections.
Watch task does not have execution section, but this info can be added when provider resolves the task.
Video: https://youtu.be/-aHEBwc_x3s

[azatsarynnyy]

FYI: using _ as a prefix for the private properties is against the coding guidelines https://github.com/theia-ide/theia/wiki/Coding-Guidelines#names
Please, check the properties in other classes.

[RomanNikitenko]

@azatsarynnyy thank you, I will check!