[Float][Fizz][Static] add `importMap` option to Fizz and Static server renderers (#27260)

Import maps need to be emitted before any scripts or preloads so the
browser can properly locate these resources.

Unlike most scripts, importmaps are singletons meaning you can only have
one per document and they must appear before any modules are loaded or
preloaded. In the future there may be a way to dynamically add more
mappings however the proposed API for this seems likely to be a
javascript API and not an html tag.

Given the unique constraints here this PR implements React's support of
importMaps as the following

1. an `importMap` option accepting a plain object mapping module
specifier to path is accepted in any API that renders a preamble (head
content). Notably this precludes resume rendering because in resume
cases the preamble should have already been produced as part of the
prerender step.
2. the importMap is stringified and emitted as a `<script
type="importmap">...</script>` in the preamble.
3. the importMap is escaped identically to how bootstrapScriptContent is
escaped, notably, isntances of `</script>` are escaped to avoid breaking
out of the script context

Users can still render importmap tags however with Float enabled this is
rather pointless as most modules will be hoisted above the importmap
that is rendered. In practice this means the only functional way to use
import maps with React is to use this config API.