…stom values

Because `and` and `or` are short-circuiting operations in Nushell, they must be compiled to a sequence that avoids evaluating the RHS if the LHS is already sufficient to determine the output - i.e., `false` for `and` and `true` for `or`. I initially implemented this with `branch-if` instructions, simply returning the RHS if it needed to be evaluated, and returning the short-circuited boolean value if it did not.

Example for `$a and $b`:

```
   0: load-variable          %0, var 999 "$a"
   1: branch-if              %0, 3
   2: jump                   5
   3: load-variable          %0, var 1000 "$b" # label(0), from(1:)
   4: jump                   6
   5: load-literal           %0, bool(false) # label(1), from(2:)
   6: span                   %0          # label(2), from(4:)
   7: return                 %0
```

Unfortunately, this broke polars, because using `and`/`or` on custom values is perfectly valid and they're allowed to define that behavior differently, and the polars plugin uses this for boolean masks. But without using the `binary-op` instruction, that custom behavior is never invoked. Additionally, `branch-if` requires a boolean, and custom values are not booleans. This changes the IR to the following, using the `match` instruction to check for the specific short-circuit value instead, and still invoking `binary-op` otherwise:

```
   0: load-variable          %0, var 125 "$a"
   1: match                  (false), %0, 4
   2: load-variable          %1, var 124 "$b"
   3: binary-op              %0, Boolean(And), %1
   4: span                   %0          # label(0), from(1:)
   5: return                 %0
```

I've also renamed `Pattern::Value` to `Pattern::Expression` and added a proper `Pattern::Value` variant that actually contains a `Value` instead. I'm still hoping to remove `Pattern::Expression` eventually, because it's kind of a hack - we don't actually evaluate the expression, we just match it against a few cases specifically for pattern matching, and it's one of the cases where AST leaks into IR and I want to remove all of those cases, because AST should not leak into IR.

Fixes nushell#14518