[CLI] Load custom PHP.wasm extensions

[adamziel]

What it does

Adds Playground CLI support for loading custom PHP.wasm extensions before PHP
starts:

wp-playground server \
	--php=8.4 \
	--php-extension=./dist/wp_mysql_parser/manifest.json

--php-extension is repeatable and accepts a local path, file: URL, or
http(s): URL to a PHP extension manifest.

Manifests can now describe both the artifact matrix and startup settings in one
JSON file:

{
	"name": "spx",
	"version": "0.1.0",
	"artifacts": [
		{
			"phpVersion": "8.4",
			"sourcePath": "spx-php8.4-jspi.so"
		}
	],
	"iniEntries": {
		"spx.http_enabled": "1"
	},
	"env": {
		"SPX_DATA_DIR": "/internal/shared/spx/data"
	}
}

Rationale

The PHP.wasm runtime can load externally built JSPI side modules, and
@php-wasm/compile-extension emits manifests for those builds. CLI users still
had no direct way to pass those manifests through to the Node runtime.

Custom extensions must be declared at startup because PHP reads extension
.ini files while booting. Passing manifests through the existing runtime
extensions array keeps CLI behavior aligned with loadNodeRuntime().

The user-facing format is intentionally one manifest file rather than separate
manifest and loader-config files. Runtime settings such as iniEntries, env,
loadWithIniDirective, and extensionDir live directly in the manifest.

Implementation

• Renames the CLI extension adapter to cliExtensionArgsToExtensionsArray().
• Combines built-in extension flags (intl, redis, memcached, xdebug)
  with custom --php-extension=<manifest> entries.
• Threads the combined PHPExtension[] into both Blueprint v1 and v2 CLI
  workers.
• Extends the PHP extension manifest type and generated schema/validator to
  allow runtime startup settings.
• Applies manifest-declared runtime settings in resolvePHPExtension().
• Documents the single manifest-based CLI path in the CLI and
  @php-wasm/compile-extension READMEs.

Testing instructions

npm exec -- nx run playground-cli:test-playground-cli --testFiles=tests/php-extensions.spec.ts
npm exec -- nx run php-wasm-universal:test:vite --testFile=load-extension.spec.ts
npm exec -- nx run playground-cli:typecheck
npm exec -- nx run php-wasm-universal:typecheck
npm exec -- nx run playground-cli:lint
npm exec -- nx run php-wasm-universal:lint
npm exec -- nx run php-wasm-compile-extension:typecheck
npm exec -- nx run php-wasm-compile-extension:lint
git diff --check