Refactor SVG inlining from md-icon into cdk-svg

The SVG inlining features of md-icon are more generally useful than for just displaying icons. We should refactor the code such that:
* The SVG portions of `MdIconRegistry` are moved to `IconRegistry` in the cdk, with `MdIconRegistry` then extending `IconRegistry`
* The SVG inlining code in `md-icon` is moved into `cdk-svg` with `md-icon` then consuming `cdk-svg`.

This will enable us to add additional APIs to `cdk-svg` around customizing `viewBox` and `preserveAspectRatio` (see https://css-tricks.com/scale-svg)

Notes:
* Part of this should be moving SVG parsing (i.e. `string` -> `SVGElement`) into its own utility that can be mocked. This will be useful when ensuring that parsing only happens when expected (since it can have performance impacts if overdone).

找到 MdIconRegistry 当前的 SVG 部分以及 md-icon 使用的 SVG 内联代码，然后将它们与 cdk IconRegistry 和 cdk-svg 入口进行比较。当 SVG 注册表行为和内联由 CDK 提供、MdIconRegistry 扩展 IconRegistry、md-icon 使用 cdk-svg，并且 SVG 解析被隔离到可模拟的实用工具中时，重构就完成了。

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