internal/filetypes: avoid using CUE at runtime

[rogpeppe]

The internal/filetypes package currently uses CUE itself to decide what attributes should apply
for any given file extension and user-supplied filetype hints. This is a lovely and expressive
way of implementing the logic, but comes with certain down sides:

1. it's somewhat slow: the generality of CUE means that the logic is never going to be as fast as hard-coded rules
2. it can easily lead to cyclic import dependencies. It means that some parts of the internals of CUE cannot depend on internal/filetypes because that in turn depends on the entire CUE standard library which has many dependencies. This should really be a low-dependency leaf package.

Point 1 might be addressable with performance improvements, but point 2 is a significant wart.

There are at least two possible ways to work around this:

• rewrite the logic in Go
• use a preprocessor to evaluate the CUE and generate Go code or data structures that encapsulate the logic