Fix gopls replace_symbol_body corrupting type/var/const declarations

gopls reports the document-symbol range of single type/var/const
declarations starting at the identifier (after the keyword), unlike func
declarations whose range includes the func keyword. Serena derives both
the displayed body and the replacement range from that range, so the
keyword was dropped from both for these declarations and a
keyword-inclusive edit corrupted the file (type Foo became type type Foo).

Override request_document_symbols in gopls to extend the range start back
to the keyword when only the keyword and indentation precede the
identifier, recomputing the body so it stays consistent with the
replacement range. Funcs, grouped declarations, and selectionRange are
left untouched.

Adds a unit test for the symbol body and range plus an end-to-end
replace_symbol_body round-trip regression test.

test/resources/repos/go/test_repo/symbol_body.go

@@ -0,0 +1,26 @@
+package main
+
+// BodyStruct is a single type declaration used to verify that replace_symbol_body
+// includes the leading `type` keyword in the symbol body and replacement range.
+type BodyStruct struct {
+	Value int
+}
+
+// NamedInt is a single defined-type declaration (non-struct).
+type NamedInt int
+
+// AliasInt is a type alias declaration.
+type AliasInt = int
+
+// GlobalCounter is a single package-level var declaration.
+var GlobalCounter int = 0
+
+// MaxItems is a single package-level const declaration.
+const MaxItems = 100
+
+// GroupedA and GroupedB are declared in a grouped var block, where the `var`
+// keyword is on a separate line and must NOT be folded into either symbol body.
+var (
+	GroupedA int    = 1
+	GroupedB string = "two"
+)