"Default Block" / more than one match per line for switch statement

Two separate feature requests

1. Have a default block available for switch statements (where if none of the is block match, it will execute the default block)
e.g.
```scala
switch(value) {
is(1) {
// do something
}
is(2) {
// do something
}
default {
error := true.B
assert(0)
}
}
```

2. Allow more than one matched value for each is block match.
e.g. (the first two cases are more useful, the subsequent examples have potential usages, but I haven't encountered it yet)
```scala
switch(value) {
is(1) {
// do something
}
is(2, 4) {
// do something
}
// we might need to figure out what it means if the user duplicate a value in this case (perhaps
execute both is blocks?) Or just error out and disallow it if it overlaps.
is(4, 6) {
}
// or maybe even more fancy, but can no longer check for non-overlapping matches if that's a requirement
is(>5) {
}
// Or even generalizing it by allowing a predicate function (Maybe this and the previous is too much, since if it gets that fancy, should probably just use when statement, and feature not in verilog anyway)
is( (v: Int):Bool => { v<7 }) {
}
}
```