SSR: Add wp-context, wp-bind, wp-class, wp-style (#133)

Implement Server-side rendering of those directives via `WP_HTML_Tag_Processor`.

Gutenberg 15.0 is the first version to include [`get_attribute_names_with_prefix()`](WordPress/gutenberg#46840). This means that we now have everything we need to implement `wp-bind`, `wp-class`, and `wp-style`. This PR thus adds those directives, plus `wp-context`.

This also adds PHP unit test infrastructure and CI, in order to allow us to adopt a test-driven development approach for individual directives. Those "infrastructure" files are mostly copied directly from GB, with some minor modifications. Refer to individual commit messages on the PR to learn more.

.github/workflows/unit-test.yml

@@ -0,0 +1,46 @@
+name: Unit Tests
+
+on:
+    pull_request:
+    push:
+        branches:
+            - 'main*'
+
+# Cancels all previous workflow runs for pull requests that have not completed.
+concurrency:
+    # The concurrency group contains the workflow name and the branch name for pull requests
+    # or the commit hash for any other events.
+    group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+    cancel-in-progress: true
+
+jobs:
+    unit-php:
+        name: PHP
+        runs-on: ubuntu-latest
+        if: ${{ github.repository == 'WordPress/block-hydration-experiments' || github.event_name == 'pull_request' }}
+
+        steps:
+            - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8 # v3.1.0
+
+            - name: Install Composer dependencies
+              run: |
+                  composer install
+
+            - name: Use desired version of NodeJS
+              uses: actions/setup-node@8c91899e586c5b171469028077307d293428b516 # v3.5.1
+              with:
+                  node-version: 14
+                  cache: npm
+
+            - name: Npm install and build
+              run: |
+                  npm ci
+                  npm run build
+
+            - name: Run WordPress
+              run: |
+                  npm run wp-env start
+
+            - name: Running single site unit tests
+              run: npm run test:unit:php
+              if: ${{ success() || failure() }}

.wp-env.json

@@ -5,5 +5,12 @@
 	],
 	"config": {
 		"SCRIPT_DEBUG": true
+	},
+	"env": {
+		"tests": {
+			"mappings": {
+				"wp-content/plugins/block-hydration-experiments": "."
+			}
+		}
 	}
 }

composer.json

@@ -29,7 +29,9 @@
 		"squizlabs/php_codesniffer": "^3.5",
 		"phpcompatibility/phpcompatibility-wp": "^2.1.3",
 		"wp-coding-standards/wpcs": "^2.2",
-		"sirbrillig/phpcs-variable-analysis": "^2.8"
+		"sirbrillig/phpcs-variable-analysis": "^2.8",
+		"phpunit/phpunit": "^9.5",
+		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"require": {
 		"composer/installers": "~1.0"