Update FormattedValue.postinit and its brain

[DanielNoord]

Type of Changes

	Type
✓	🐛 Bug fix

Description

Refs #1516.

Decided to make this a breaking change as it just so much easier to update signatures when these are keyword only. I don't think a small breaking change at the top of the signature matters too much.

Using an Enum doesn't make a lot of sense as the ast module has it as an int. So we would need to of some casting to that Enum and that could be the source of nasty crashes,

[codecov]

Codecov Report

Merging #2029 (be2d39e) into main (d57e34f) will increase coverage by 0.00%.
The diff coverage is 81.81%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2029   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files          95       95           
  Lines       11000    11003    +3     
=======================================
+ Hits        10206    10209    +3     
  Misses        794      794           

Flag	Coverage Δ
linux	92.54% <81.81%> (+<0.01%)	⬆️
pypy	88.50% <81.81%> (+<0.01%)	⬆️
windows	92.33% <81.81%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
astroid/rebuilder.py	98.29% <ø> (ø)
astroid/brain/brain_fstrings.py	36.66% <77.77%> (+7.03%)	⬆️
astroid/nodes/node_classes.py	94.87% <100.00%> (ø)

[Pierre-Sassoulas]

We should probably cover what's not currently covered ?

[DanielNoord]

Shall we do that in a follow up? Functionally nothing changes here, other than conversion now being passed correctly in the brain were it was previously just dropped. This whole brain seems to be untested and I don't want to block this PR on me having time to look at the testing of the brain.