ruleguard,dsl: add Var.Node filters

[quasilyte]

Add the ability to assert the submatch node AST type.

A new Var.Node.Is() function accepts the type name string,
like Ident (ast.Ident), Expr (ast.Expr) and so on.
For any unexpected arguments the rule compilation will fail.

We may add more node categories that are not declared in
go/ast package for our convenience.

Rationale: sometimes we need to check not only the types,
but also the "shape" of the match. For example, we may want
to enforce some func arguments to be literal strings.
Right now we can only do this by doing a regexp check
that may look like <broken markdown> but that's not reliable, nor efficient.
What we can do is to add a negated *ast.BasicLit constraint,
optionally with a type check if func signature accepts interface{}.

// Match any non-literal $s
Where(!m["s"].Node.Is(`BasicLit`))

Note that m["s"].Const is not identical here. It would accept
const expressions like "a" + "b" as well.

Not to be confused with an actual type. It's just a
syntactical check whether a matched node belongs to some
syntax category as defined by the go/ast package.

Signed-off-by: Iskander Sharipov [email protected]

[quasilyte]

https://go-ruleguard.github.io/by-example/node-predicates