`lfortran fmt` should use line continuation for long lines

Original issue: https://gitlab.com/lfortran/lfortran/-/issues/340

A user defined maximum line length (say 80) should be respected by correctly inserting line continuation & character. A quick way to achieve that is by:

diff --git a/src/lfortran/ast_to_src.cpp b/src/lfortran/ast_to_src.cpp
index 27452f23..3a4805e4 100644
--- a/src/lfortran/ast_to_src.cpp
+++ b/src/lfortran/ast_to_src.cpp
@@ -877,10 +877,20 @@ public:
             }
             if (x.n_syms > 0) {
                 r.append(" :: ");
+                int column = r.size();
                 for (size_t i=0; i<x.n_syms; i++) {
                     visit_var_sym(x.m_syms[i]);
+                    if (column + s.size() > 75) {
+                        r += " &\n";
+                        r += indent + "    ";
+                        column = indent.size() + 4;
+                    }
                     r += s;
-                    if (i < x.n_syms-1) r.append(", ");
+                    column += s.size();
+                    if (i < x.n_syms-1) {
+                        r.append(", ");
+                        column += 2;
+                    }
                 }
             }
         }

But it seems quite messy and error prone. A cleaner (perhaps a bit slower) approach would be for the visit methods to not return a string in s but rather a list of tokens (a list of strings), a token can be a new line and an indentation also. Then could have a class that handles converting this list of tokens into a string, which would keep track of indentation and the current column and insert continuation character & appropriately.

Start with the formatter implementation in src/lfortran/ast_to_src.cpp and trace how the lfortran fmt command assembles output strings and indentation. Compare the proposed line-length handling with the cleaner token-based design described in the issue. Done means user-defined maximum line lengths are respected with valid Fortran continuation characters without breaking formatted output.