Add composer.json

[kabalin]

This is to make possible installation using composer, which support moodle plugin types nowadays.

Quick intro:

• "type": "moodle-format" make plugin install into course/format/<name> dir
• "installer-name": "flexsections" makes installed dir name flexsections rather than marinaglancy/moodle-format_flexsections
• the rest is self explanatory

Once merged, publishing on packagst it required. It traces tags in repo, so no further steps after initial publishing will be required, all further releases will be published automatically.