Cover flatshapes (#35)

All right! That took some time 🕙. Sorry for the long wait, I had _life_
coming in the middle.
Anywho, I finished a bit more of `FlatShape`s
- `FlatShape::StringInterpolation`
- `FlatShape::Block`
- `FlatShape::Closure`
- `FlatShape::InternalCall(declid)`
- `FlatShape::External`
- `FlatShape::ExternalArg`
- `FlatShape::Signature`
- `FlatShape::Keyword`
- `FlatShape::VarDecl(varid)`
- `FlatShape::Variable`

There are a bunch more to solve, but most importantly, the code is
screaming for a refactor.
The issue is that I used more and more booleans to do quick checks "is
this coming from a `def` keyword?" "are we in a string interpolation
line?" and so on, that it gets _very_ spaguetti if we dont use smarter
solutions like `peek` for example

As a conclusion, I am ready for this PR, but I know now that the
direction is not to include all the remaining `FlatShape`s, but
reorganize the code a little better 😇

---------

Co-authored-by: amtoine <[email protected]>
Co-authored-by: Auca Maillot <[email protected]>
Co-authored-by: Antoine Stevan <[email protected]>

.gitignore

@@ -6,3 +6,7 @@ result
 
 # Direnv
 .direnv
+
+# editors
+.idea/
+.vscode/