PS: Handle more flow sources

[MathiasVP]

This PR does a couple of things to add more flow sources to PowerShell:

• It adds a Stored.qll file containing a subclass of local flow sources that represents file reads. It turns out we already have some flow sources belonging to this kind since we stole them from C#:

  codeql/powershell/ql/lib/semmle/code/powershell/frameworks/SystemIOFile/model.yml

  Line 10 in 1637df0

  - ["system.io.file!", "Method[Open].ReturnValue", "file"]

  . We just didn't have a QL class to make them part of SourceNode 😅

• Then, I also discovered two sources of user flow recently. For example, Select-XML, which can read user input when given an -Path argument. So this PR also adds MaD syntax for specifying "the return value of a call when there is a specific named argument". After adding this functionality, this PR also models calls to Select-XML and Format-Hex as remote flow sources of this kind.

• It fixes a bunch of incorrect models for flow sources. I had incorrectly marked some of them as static method calls, where they should have been non-static method calls.

• Field/Propertys should have been Member all along in the type models. I've done a mass-renaming to fix this. I've also made all the method names lower-case.

• It adds testcases for all the currently flow sources. Most of the above fixes are motivated by these tests which were written by a combination of me and Bing 😅

[chanel-y]

whoops opened the window but then forgot to approve mb