Anonymous variable _ is allowed outside of patterns

[rodinaarssen]

Describe the bug

While _ is a valid identifier, it should not be allowed outside of patterns (e.g., as an expression, in an assignment, in a declaration). If assigned to, it gets a value, although it then still behaves "normally" in the context of a pattern.

To Reproduce

rascal>_ = 1;
int: 1
rascal>_
1
ok
rascal>_ := 2 && <_, _> := <"", |hi:///|>
bool: true