backport: fix(:source, nvim_exec): handle Vimscript line continuations #14809

Problem:
Anonymous :source (no args) and nvim_exec() don't support Vimscript line continuations.

Solution:
Factor out the concat logic into concat_continued_line() and a
CONCAT_CONTINUED_LINES macro for simple concatenations where lines are
fetched individually.

Closes #14807

test/functional/ex_cmds/source_spec.lua

@@ -8,6 +8,8 @@ local feed = helpers.feed
 local feed_command = helpers.feed_command
 local write_file = helpers.write_file
 local exec = helpers.exec
+local exc_exec = helpers.exc_exec
+local exec_lua = helpers.exec_lua
 local eval = helpers.eval
 local exec_capture = helpers.exec_capture
 local neq = helpers.neq
@@ -18,16 +20,30 @@ describe(':source', function()
   end)
 
   it('current buffer', function()
-    insert('let a = 2')
+    insert([[
+      let a = 2
+      let b = #{
+        \ k: "v"
+       "\ (o_o)
+        \ }]])
+
     command('source')
     eq('2', meths.exec('echo a', true))
+    eq("{'k': 'v'}", meths.exec('echo b', true))
+
+    exec('set cpoptions+=C')
+    eq('Vim(let):E15: Invalid expression: #{', exc_exec('source'))
   end)
 
   it('selection in current buffer', function()
-    insert(
-      'let a = 2\n'..
-      'let a = 3\n'..
-      'let a = 4\n')
+    insert([[
+      let a = 2
+      let a = 3
+      let a = 4
+      let b = #{
+       "\ (>_<)
+        \ K: "V"
+        \ }]])
 
     -- Source the 2nd line only
     feed('ggjV')
@@ -38,13 +54,17 @@ describe(':source', function()
     feed('ggjVG')
     feed_command(':source')
     eq('4', meths.exec('echo a', true))
+    eq("{'K': 'V'}", meths.exec('echo b', true))
+
+    exec('set cpoptions+=C')
+    eq('Vim(let):E15: Invalid expression: #{', exc_exec("'<,'>source"))
   end)
 
   it('multiline heredoc command', function()
-    insert(
-      'lua << EOF\n'..
-      'y = 4\n'..
-      'EOF\n')
+    insert([[
+      lua << EOF
+      y = 4
+      EOF]])
 
     command('source')
     eq('4', meths.exec('echo luaeval("y")', true))
@@ -67,13 +87,21 @@ describe(':source', function()
       vim.g.b = 5
       vim.g.b = 6
       vim.g.b = 7
+      a = [=[
+       "\ a
+        \ b]=]
     ]])
 
     command('edit '..test_file)
+
     feed('ggjV')
     feed_command(':source')
-
     eq(6, eval('g:b'))
+
+    feed('GVkk')
+    feed_command(':source')
+    eq('   "\\ a\n    \\ b', exec_lua('return _G.a'))
+
     os.remove(test_file)
   end)
 
@@ -84,12 +112,16 @@ describe(':source', function()
       vim.g.c = 10
       vim.g.c = 11
       vim.g.c = 12
+      a = [=[
+        \ 1
+       "\ 2]=]
     ]])
 
     command('edit '..test_file)
     feed_command(':source')
 
     eq(12, eval('g:c'))
+    eq('    \\ 1\n   "\\ 2', exec_lua('return _G.a'))
     os.remove(test_file)
   end)