Add cast and isinstance keywords

[tetromino]

Finishing support for the type syntax proposed in bazelbuild/starlark#362

After some reflection, I think we should always parse cast/isinstance as keywords (requiring 2 arguments, one of which is a value expression and the other a type expression) since permitting pre-type Starlark syntax is likely to only lead to confusion, and would be quite difficult to get right.

Note that this is an incompatible change: forms like cast(x, y, z) or cast = foo will now be a parse error. However, in practice, I observe no occurrences of cast or isinstance used as identifiers in any Starlark code in Google's monorepo, so I believe that it's rather unlikely to break anyone.

Store the parsed cast/isinstance exprs as CallExpr-s (to avoid diverging their formatting/walking/etc. support from call expressions) with an enum field to mark that they are of a special kind.

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• The code in this PR is covered by unit/integration tests.
• I have tested these changes and provide testing instructions below.
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[fmeum]

Needs a rebase after the Buck2 fix

[fmeum]

I unfortunately missed that this didn't end up merging for v10. I will send a v10.0.2 with this and argue for it to be a bugfix for type support, but call out the incompatible change.

[fmeum]

@tetromino I rebased the PR onto main and will merge it now.