bracket-push: renamed to matching-brackets

[petertseng]

In exercism/problem-specifications#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 exercism/problem-specifications#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.