Resolve merge conflicts with devel branch refactoring (#25423)

The PR branch had merge conflicts with `devel` due to a major compiler
refactoring that extracted type definitions from `compiler/ast.nim` into
a new `compiler/astdef.nim` file.

## Changes

- Resolved conflict in `compiler/ast.nim` by accepting `devel`'s
refactored structure
- Merged 763 commits from `devel` branch (commit range:
`ce6a345..b3273e7`)
- Preserved original PR changes removing deprecated symbols from
`lib/core/macros.nim`

The core PR functionality (removal of deprecated macros API since
v0.18.1) remains intact while incorporating the upstream AST
refactoring.

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testament/categories.nim

@@ -493,8 +493,8 @@ proc icTests(r: var TResults; testsDir: string, cat: Category, options: string;
     tooltests = ["compiler/nim.nim"]
     writeOnly = " --incremental:writeonly "
     readOnly = " --incremental:readonly "
-    incrementalOn = " --incremental:on -d:nimIcIntegrityChecks "
-    navTestConfig = " --ic:on -d:nimIcNavigatorTests --hint:Conf:off --warnings:off "
+    incrementalOn = " --incremental:legacy -d:nimIcIntegrityChecks "
+    navTestConfig = " --ic:legacy -d:nimIcNavigatorTests --hint:Conf:off --warnings:off "
 
   template test(x: untyped) =
     testSpecWithNimcache(r, makeRawTest(file, x & options, cat), nimcache)
@@ -508,7 +508,7 @@ proc icTests(r: var TResults; testsDir: string, cat: Category, options: string;
 
   template checkTest() =
     var test = makeRawTest(file, options, cat)
-    test.spec.cmd = compilerPrefix & " check --hint:Conf:off --warnings:off --ic:on $options " & file
+    test.spec.cmd = compilerPrefix & " check --hint:Conf:off --warnings:off --ic:legacy $options " & file
     testSpecWithNimcache(r, test, nimcache)
 
   if not isNavigatorTest:

testament/important_packages.nim

@@ -93,7 +93,7 @@ pkg "lockfreequeues"
 pkg "loopfusion"
 pkg "macroutils"
 pkg "manu"
-pkg "markdown"
+pkg "markdown", "nim c -r tests/testmarkdown.nim"
 pkg "measuremancer", "nimble testDeps; nimble -y test"
 pkg "memo"
 pkg "metrics"