This PR is a follow up to #64092, which fixed important issues but it was implemented in an overly complex and inefficient way (because it forced the default code path to always go through string operations).
This cleans up all the shader parameter code.
This fixes#54336. Also fixes#56219 because, as the new code never queries the RenderingServer on load, potential deadlocks are avoided.
**NOTE**: materials saved between #62972 and #64092 will no longer work and will need to be resaved in an earlier version.
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