Implement support for RatOS

[depau]

(note: set to draft since I noticed some issues after submitting, sorry about that!)

RatOS is a distribution of Klipper with some heavy customizations, used for RatRig V-Core printers, also compatible with some Vorons and Prusas.

With RatRig V-Core 4 printers in particular it's really not possible to use stock Klipper since a lot of features are missing, such as the IDEX functionality and tuning helpers.

I use this tool regularly with my V-Core 4, but each time I use it I need to edit the G-Code manually to make sure RatOS is happy.

This is what needs to happen for RatOS to be happy:

• The G-Code must start with a comment declaring the generator version. RatOS will only accept G-Code from Prusa Slicer, SuperSlicer and Orca Slicer, as well as G-Code that declares to be made for RatOS. This is because RatOS post-processes G-Code to support IDEX copy and mirror modes (and possibly more reasons).
  • A generator that reports compatibility with RatOS must report v0.1 due to an oversight: Parse RatOS dialect version when used Rat-OS/RatOS-configurator#91
  • EDIT: it turns out that while the oversight is real, this still doesn't work since RatOS tries to parse the version number as SemVer, and 0.1 is not valid SemVer. Instead, I implemented simply spoofing a Prusa Slicer version.
• The version header must be at the beginning of the file, it can't be placed in the start gcode.
• Unlike regular Klipper, RatOS uses T0/T1 to select the toolhead.
• T1 should appear after the start g-code since START_PRINT does a lot of stuff; the pre-processor decides which heaters to turn on based on its understanding of the G-Code.
• T1 must not be followed by a comment, or else some regex in the pre-processor will blow up
• The start macro should have a TOTAL_LAYER_COUNT parameter or else it will complain.
• The start macro must report the min/max X/Y coordinates of the print: newer RatRig printers use a Beacon Contact probe; to speed up print setup times RatOS will only scan the build plate over the print area.

Additionally, this PR:

• Fixes a simple syntax issue with selectors
• Forcefully disables homing/heating G-Codes in the preamble for RatOS since START_PRINT takes care of that with over-complicated logic.

Let me know if you need any changes, and thanks for this tool!