Named arguments in commands + many grammar fixes (#20994)

* Breaking parser changes, implement nim-lang/RFCs#442

Types are separated from expressions and better reflected in the grammar.

* add test

* more accurate grammar

* fix keyword typedescs

* accept expressions in proc argument lists

* CI "fixes"

* fixes

* allow full ref expressions again, adapt old tests

* cleanup, fix some tests

* improve grammar, try and revert semtypes change

* restrict sigil binding to identOrLiteral

* fix, should have caught this immediately

* add changelog entry, fix double not nil bug

* correct grammar

* change section

* fix

* real fix hopefully

* fix test

* support LL(1) for tuples

* make grammar.txt too

changelog.md

@@ -80,6 +80,11 @@
 - Removed two type pragma syntaxes deprecated since 0.20, namely
   `type Foo = object {.final.}`, and `type Foo {.final.} [T] = object`.
 
+- `foo a = b` now means `foo(a = b)` rather than `foo(a) = b`. This is consistent
+  with the existing behavior of `foo a, b = c` meaning `foo(a, b = c)`.
+  This decision was made with the assumption that the old syntax was used rarely;
+  if your code used the old syntax, please be aware of this change.
+
 - [Overloadable enums](https://nim-lang.github.io/Nim/manual.html#overloadable-enum-value-names) and Unicode Operators
   are no longer experimental.