Dump all UWP logging output to a file

[jdm]

These changes ensure that we keep getting debug output in the VS output window, but we also write each message to a file in the app's local data directory. These changes also extend the C API a bit to support more generic logging facilities, removing some of the Windows-specific nature of the VSLogger. Fixes part of #23813.

[jdm]

This is ready for review. I gated the log output on debug builds by default to avoid the risk of hurting perf by writing to a file.

[Manishearth]

@bors-servo r+

[Manishearth]

@bors-servo r+

[bors-servo]

📌 Commit 9fdb420 has been approved by Manishearth

[bors-servo]

⌛ Testing commit 9fdb420 with merge b4aea41...

[bors-servo]

💔 Test failed - status-taskcluster

[jdm]

@bors-servo r=Manishearth

[bors-servo]

📌 Commit b1b4b5e has been approved by Manishearth

[bors-servo]

⌛ Testing commit b1b4b5e with merge cc78171...

[bors-servo]

💔 Test failed - status-taskcluster

[Manishearth]

 unused variable: `handler`
  --> ports/libsimpleservo/capi/src/lib.rs:70:27
   |
70 | fn redirect_stdout_stderr(handler: LogHandlerFn) -> Result<(), String> {
   |                           ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_handler`
   |
   = note: `-D unused-variables` implied by `-D warnings

[jdm]

@bors-servo r=Manishearth

[bors-servo]

📌 Commit 6a73c5b has been approved by Manishearth

[bors-servo]

⌛ Testing commit 6a73c5b with merge aa37904...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: Manishearth
Pushing aa37904 to master...