pw_presubmit: List steps in JSON not plain text
pw_presubmit: List steps in JSON not plain text

When called with --only-list-steps, list them in JSON instead of as
plain text. This will allow the format to be extended without requiring
obscure syntax.

Bug: b/253021172
Change-Id: Ia4bb02340c61f2dc744e1308c05e28045621bde4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/114131
Commit-Queue: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Reviewed-by: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed