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/api/vim_spec.lua

@@ -115,6 +115,19 @@ describe('API', function()
       nvim('exec','autocmd BufAdd * :let x1 = "Hello"', false)
       nvim('command', 'new foo')
       eq('Hello', request('nvim_eval', 'g:x1'))
+
+      -- Line continuations
+      nvim('exec', [[
+        let abc = #{
+          \ a: 1,
+         "\ b: 2,
+          \ c: 3
+          \ }]], false)
+      eq({a = 1, c = 3}, request('nvim_eval', 'g:abc'))
+
+      -- try no spaces before continuations to catch off-by-one error
+      nvim('exec', 'let ab = #{\n\\a: 98,\n"\\ b: 2\n\\}', false)
+      eq({a = 98}, request('nvim_eval', 'g:ab'))
     end)
 
     it('non-ASCII input', function()