🏗✨ Check non-JS files for formatting errors using prettier during local and Travis PR checks

[rsimha]

PR highlights:

• Adds a new task gulp prettify to check non-JS files for formatting errors using prettier
• Adds a new task gulp check-owners to check OWNERS files for correctness (with a placeholder check until [owners] implement blocking Travis check to validate modified OWNERS.yaml syntax amp-github-apps#281 is fixed)
• Removes the old task gulp json-syntax since it will be replaced by gulp prettify for .json files
• Renames build-system/tasks/json-syntax.js to build-system/tasks/caches-json.js now that it only checks caches.json for correctness
• Updates the build target logic used by the local and Travis PR check workflow to account for OWNERS files (so all tests aren't run for PRs that only touch OWNERS files)
• Adds the newly introduced checks to the local and Travis PR check workflows
• Moves some common functionality used by tasks like lint, prettify, etc. to a new file build-system/common/utils.js

Coming up:

• Add **/*.json and all json / json5 dotfiles to the list of files that are checked by gulp prettify (with auto-fixes) (🏗 Enable gulp prettify on several non-JS files (and auto-fix them) #25085)
• Add a --local_changes mode to gulp prettify (requires further refactoring of lint.js and prettify.js) (🏗 Add a --local_changes mode to gulp prettify #25093)
• Replace the rudimentary check in gulp check-owners with a call to the owners bot's parsing API ([owners] implement blocking Travis check to validate modified OWNERS.yaml syntax amp-github-apps#281)

Usage:

# Check all non-JS files (that can be formatted with prettier)
gulp prettify

# Check and fix all non-JS files
gulp prettify --fix

# Check a subset of non-JS files
gulp prettify --files "<glob>"

# Check all `OWNERS` files
gulp check-owners

# Check and fix all OWNERS files
gulp check-owners --fix

# Check a subset of OWNERS files
gulp check-owners --files "build-system/**/OWNERS"

Fixes #24936
Fixes #24942

[rcebulko]

Awesome! Thanks for figuring this out. Left a couple comments, approving to unblock

[rsimha]

Addressed PR comments.

[rsimha]

@estherkim I've rewritten this PR based on our discussion. The gulp prettify task will check for formatting (and optionally fix), while the gulp check-owners task can be used to check OWNERS files for correctness.

[rsimha]

@rcebulko All comments addressed.

[rcebulko]

One comment re: [].forEach(async .. => ...), approving to unblock

[estherkim]

Thank you for the work on this! And thanks for clarifying the changes in the PR description.

[rsimha]

Thanks for the reviews. Looks like the latest rebase caught some bad formatting: https://travis-ci.org/ampproject/amphtml/jobs/598788352#L412

I'll rebase, fix, and check this in once Travis is green.