PS: Fix more Chanel-reported PowerShell issues

[MathiasVP]

This PR fixes two three(!) PowerShell issues reported by @chanel-y:

Missing edges in API graphs

Consider this example:

$executionContext.InvokeCommand.InvokeScript(...)

@chanel-y reported that the following use of API graphs didn't work:

from API::Node call
where
  API::getTopLevelMember("executioncontext").getMember("invokecommand").getMethod("invokescript") =
    call
select call, "..."

This was because we were missing two API edges:

• An edge from the top level (i.e., root) node to any use of an automatic variable such as $executionContext, and
• An edge from a qualifier to a member read such as $executionContext.InvokeCommand

With those two extra steps added, the above snippet works 🥳

In addition, I added two drive-by fixes:

• The first commit adds a location to the synthesized top-level function
• The last commit adds a call to toLowerCase when selecting a method using getMethod. This is, again, necessary because PowerShell is case insensitive, and $executionContext.InvokeCommand.InvokeScript and $executionContext.invokecommand.invokescript both invokes the InvokeScript method.

Missing pipeline parameters

Recall that these two snippets are equivalent:

param([Parameter(ValueFromPipeline = $true)] $arg)

process {
    $arg
}

and

process {
    $_
}

One has an explicit pipeline parameter, and the other has an implicit one. That means that we sometimes have to synthesize implicit pipeline parameters in the AST.

This was already done in #178 and #181. However, sometimes I referred to a non-existing explicitly-provided pipeline parameter, and other times I referred to a synthesized implicitly-provided pipeline parameter when I should be referring to the explicitly-provided one. This PR makes sure that we always:

• Refer to the synthesized one only if no explicit one is provided, and
• Always refer to the explicit one if such a parameter is provided

Missing model edges in taint-tracking

We added models-as-data in #134, and slightly regressed it in #178. @chanel-y noticed this due to a lack of taint-flow through CodeGeneration.EscapeSingleQuotedStringContent. This PR adds the missing QL required to get that flow, and also adds a MaD taint-model for this method.

I'll do a follow-up that adds autogenerated models for the other methods later.