feat: add vue-vapor compiler for Vue 3.6 Vapor mode - #438

Adds a vue-vapor compiler that emits Vapor mode components instead of virtual-DOM ones:

Icons({ compiler: 'vue-vapor' })

The existing vue3 compiler emits a VDOM component, and a VDOM component in a Vapor tree doesn't fail loudly. It renders nothing at all, no error or warning. So a Vapor app currently has no working option short of hand-rolling a CustomCompiler.

Notes:

• Compiles with @vue/compiler-vapor and wraps the resulting render in defineVaporComponent, mirroring how vue3.ts wraps compileTemplate output.
• Reuses the existing handleSVGId helper, so per-instance gradient/mask ids behave exactly as with vue3. The one difference: the idMap is injected as a render-function local rather than into setup(), since module scope is shared by every instance of an icon, which is the collision handleSVGId exists to avoid.
• @vue/compiler-vapor is an optional peer dependency, same treatment as @vue/compiler-sfc.
• guessCompiler is left alone: Vapor is opted into per SFC and isn't detectable, so this stays explicit.

Linked Issues

fixes #428

Additional context

test/vueVapor.test.ts asserts invariants rather than snapshotting, since Vapor codegen is still churning across 3.6.0-rc.*.

Also verified end-to-end: mounted the generated components with createVaporApp on vue@3.6.0-rc.5 in jsdom. Two instances of a gradient icon got four distinct uicons-* ids, each url(#…) resolving to its own instance's gradient.

The part worth the most scrutiny is the string surgery on the compiled render function. It throws rather than emitting a broken module if the expected shape isn't found, but it's still coupled to pre-release codegen output. Happy to rework it if you'd prefer a different approach.