bracket-push: renamed to matching-brackets

In https://github.com/exercism/problem-specifications/pull/1501, bracket-push was renamed to matching-brackets in problem-specifications, therefore it is recommended that this track (which has the bracket-push exercise) also renames it.

The reasons for this recommendation:

to name the exercise by its story, not by what it potentially teaches
to avoid unnecessarily biasing the solution space

If this track decides to follow this recommendation, here are some things that may need a change:

config.json
name of the directory
README (contains the name of the exercise)
code

If you're not sure what needs to change, feel free to examine PRs to other tracks by looking at any PRs linked to https://github.com/exercism/problem-specifications/pull/1501.

If this track instead decides that accepting this recommendation is a bad idea for the track, then this issue should be closed upon making that decision, and none of the above steps should be taken.

exercism / scala

bracket-push: renamed to matching-brackets #607