allow defining local labels for another scope

fix gbdev#1157 for the following source code
```
section "hSAVE_locals",HRAM
func3.hSpam: ds 1  ; no longer produces an error
;.hEggs: ds 1      ; uncomment this to see the new error

section "demo",ROM0
func3:
  ldh a, [.hSpam]
  ret
```

Remove two errors:
- `Not currently in the scope of 'func3'`
- `Local label 'func3.hSpam' in main scope`

Add one error:
- `Relative local label '.hSpam' in main scope`

Add a switch to restore previous behavior in `include/asm/symbol.h`
```c
```

update tests of local label definition failure

now that local label definition is more lenient, fewer tests
should fail

begin removing issue number from test

Co-authored-by: Rangi <35663410+Rangi42@users.noreply.github.com>

treat ALLOW_LOCAL_LABEL_ANYWHERE as always on

- don't gate the new lenient behavior behind a build-time condition
- change "Relative local label" to "Unqualified local label"
  in an error message

realign tests of local labels with wrong parent

- repurpose test/asm/sym-scope from a test of producing correct
  errors to a successful test
- remove test/asm/local-wrong-parent because it is now a redundant
  successful test

based on suggestions by @Rangi42

src/asm/symbol.c

@@ -525,16 +525,14 @@ static struct Symbol *addLabel(char const *symName)
 // Add a local (`.name` or `Parent.name`) relocatable symbol
 struct Symbol *sym_AddLocalLabel(char const *symName)
 {
-	if (!labelScope) {
-		error("Local label '%s' in main scope\n", symName);
-		return NULL;
-	}
-	assert(!strchr(labelScope, '.')); // Assuming no dots in `labelScope`
+	// Assuming no dots in `labelScope` if defined
+	assert(!labelScope || !strchr(labelScope, '.'));
 
 	char fullName[MAXSYMLEN + 1];
 	char const *localName = strchr(symName, '.');
 
 	assert(localName); // There should be at least one dot in `symName`
+
 	// Check for something after the dot in `localName`
 	if (localName[1] == '\0') {
 		fatalerror("'%s' is a nonsensical reference to an empty local label\n",
@@ -546,23 +544,13 @@ struct Symbol *sym_AddLocalLabel(char const *symName)
 			   symName);
 
 	if (localName == symName) {
+		if (!labelScope) {
+			error("Unqualified local label '%s' in main scope\n", symName);
+			return NULL;
+		}
 		// Expand `symName` to the full `labelScope.symName` name
 		fullSymbolName(fullName, sizeof(fullName), symName, labelScope);
 		symName = fullName;
-	} else {
-		size_t i = 0;
-
-		// Find where `labelScope` and `symName` first differ
-		while (labelScope[i] && symName[i] == labelScope[i])
-			i++;
-
-		// Check that `symName` starts with `labelScope` and then a '.'
-		if (labelScope[i] != '\0' || symName[i] != '.') {
-			size_t parentLen = localName - symName;
-
-			assert(parentLen <= INT_MAX);
-			error("Not currently in the scope of '%.*s'\n", (int)parentLen, symName);
-		}
 	}
 
 	return addLabel(symName);

test/asm/local-without-parent.err

@@ -1,3 +1,3 @@
 error: local-without-parent.asm(2):
-    Local label '.test' in main scope
+    Unqualified local label '.test' in main scope
 error: Assembly aborted (1 error)!

test/asm/sym-scope.asm

@@ -1,8 +1,9 @@
 SECTION "Scopes", ROM0
 
-; Neither of these should be created
-.tooSoon
-Nice.try
+; Tests of injecting local labels into another label's scope.
+; This construction is useful to define a subroutine's local variables
+; in WRAM or HRAM.
+Valid.syntax
 
 Parent:
 .loc
@@ -11,8 +12,8 @@ Parent.explicit
 	dw .explicit ; This should expand to the above
 
 
-; None of the two locals below should manage to be created, being in the wrong scopes
-; Note however that `Parentheses` begins with `Parent`, which string checks may fail to handle
+; Note that `Parentheses` begins with `Parent`,
+; which string checks may fail to handle
 
 Parentheses.check
 

test/asm/sym-scope.err

@@ -1,9 +0,0 @@
-error: sym-scope.asm(4):
-    Local label '.tooSoon' in main scope
-error: sym-scope.asm(5):
-    Local label 'Nice.try' in main scope
-error: sym-scope.asm(17):
-    Not currently in the scope of 'Parentheses'
-error: sym-scope.asm(21):
-    Not currently in the scope of 'Parent'
-error: Assembly aborted (4 errors)!