[build] Make the pre-commit hook optional

Add a new NPM script, `pre-commit`, which _conditionally_ invokes
`npm test`. Previously, the pre-commit hook was configured by default to
invoke `npm test`. Now it is configured explicitly to invoke the new
`pre-commit` script.

The rationale for this change is that some folks may prefer workflows
that defer all validation until CI. Supporting these workflows is a
small change, most of it documentation.

This change isn't necessary for T257647 but supports and seems like an
overall improvement to the repo regardless of whether code is built or
not.

Bug: T257647
Change-Id: I19a78e0e677026cd6887245d96e63a414397a3db

README.md

@@ -48,5 +48,25 @@ configurable via the configurations prefixed with `VectorDefaultSkinVersion`.
 Version preference and configuration may be overridden by the `useskinversion`
 URL query parameter.
 
+### Pre-commit tests
+
+A pre-commit hook is installed when executing `npm install`. By default, it runs
+`npm test` which is useful for automatically validating everything that can be
+in a reasonable amount of time. If you wish to defer these tests to be executed
+by continuous integration only, set the `PRE_COMMIT` environment variable to `0`:
+
+```bash
+$ export PRE_COMMIT=0
+$ git commit
+```
+
+Or more succinctly:
+
+```bash
+$ PRE_COMMIT=0 git commit
+```
+
+Skipping the pre-commit tests has no impact on Gerrit change identifier hooks.
+
 ### Hooks
 See [hooks.txt](hooks.txt).

package.json

@@ -14,8 +14,10 @@
 		"selenium-test": "npm -s run test:size",
 		"doc": "jsdoc -c jsdoc.json && npm run build-storybook",
 		"build-storybook": "dev-scripts/setup-storybook.sh && build-storybook --quiet --loglevel warn -o docs/ui",
-		"svgmin": "svgo --config=.svgo.yml -q -r -f resources/"
+		"svgmin": "svgo --config=.svgo.yml -q -r -f resources/",
+		"pre-commit": "[ \"${PRE_COMMIT:-1}\" -eq 0 ] || npm -s t"
 	},
+	"pre-commit": "pre-commit",
 	"devDependencies": {
 		"@babel/core": "7.7.7",
 		"@storybook/html": "5.2.8",