PSUseDeclaredVarsMoreThanAssignments is not aware of the scriptblock context (invocation or dot-sourcing)

Steps to reproduce

Extracted from here but similar issues also reported in #711

It’s a PSSA bug. I get the same warning with:

$totalSize = 0
. { $totalSize += 1 }
$totalSize

PSSA is apparently analyzing each script block independently.

It needs to recognize when a script block is dot sourced and analyze it in the context of where it is >invoked. This isn’t always possible, but it’s usually easy to do when using ForEach-Object (which dot >sources like this).

In trying some variants on the idea, it seems like there are multiple bugs:

$totalSize = 0
function foo {
$x = $totalSize + 1
$totalSize = $x # Should warn, doesn't (bug)
}
& {
$x = $totalSize + 1
$totalSize = $x # Should warn, doesn't (bug)
}
. {
$x = $totalSize + 1
$totalSize = $x # Should not warn, doesn't (no bug)
}
. {
$totalSize += 1 # Should not warn, does (bug)
}
$totalSize

Start from the PSUseDeclaredVarsMoreThanAssignments implementation and reproduce the invocation, dot-sourcing, function, and scriptblock examples in the issue. Compare each diagnostic with the expected behavior described in the comments; done means the analyzer handles these contexts without false positives or missed warnings.