[Storage] Lazy-load git API modules

[adamziel]

What it does

Lazy-loads the git-heavy implementation modules behind the public @wp-playground/storage git APIs.

The exported functions keep the same async API surface, but git-sparse-checkout and git-create-dotgit-directory are loaded only when one of the git APIs is called.

Rationale

This avoids making every browser consumer of @wp-playground/storage pay the git implementation cost up front.

Measured with:

npm exec nx run playground-storage:build:bundle:production

Build	Initial index.js	Gzip
origin/trunk	363.40 kB	91.87 kB
This branch	25.28 kB	6.77 kB
Change	-338.12 kB	-85.10 kB

Testing instructions

npx nx run playground-storage:test
npx nx run playground-storage:build