WIP: implement jinja templated includes

[vvuk]

This is a sort of "hey, wouldn't it just work if.." WIP PR. It allows specifying variables to [include] statements, e.g.:

[include foo.cfg]
FOO: bar
THING: 2

and then foo.cfg can use jinja directives with {!% %!} {! !} (with the added !) to do templating and variable expansion that's resolved directly at include time, e.g. foo.cfg:

[somesection]
option: name-{! FOO !}
{!% if THING == 2 %!}
something: else
{!% endif %!}

The entire included file is treated as a template and rendered before being passed to the parser. Variables get maintained in an include chain -- if foo.cfg includes a file, FOO and THING will be available there too.

I still need to see how this interacts with SAVE_CONFIG (if at all). All this enables things like:

[include board-ebb36.cfg]
mcu_name: toolhead0
mcu_path: /dev/serial/....

[include board-ebb36.cfg]
mcu_name: toolhead1
mcu_path: /dev/serial/....

Checklist

• pr title makes sense
• added a test case if possible
• if new feature, added to the readme
• ci is happy and green