feat(eval/method): partially port v8.1.1921

Adds method call support for all functions in the patch, but it cannot
be fully ported due to missing tests for:

- filereadable(): requires v8.1.1378 for Test_delete_rf(), but there
                  appears to have been some trouble porting it. (#12784)

- confirm(): requires v8.1.0832 for Test_confirm() and v8.1.0815 for
             feedkeys()'s "L" flag.

             (I did attempt to port the test using nvim_input() instead,
             but seems that input handling for confirm() doesn't work in
             --headless mode?)

             Note that confirm() was actually added as a method in
             v8.1.1915.

Uncomment use of method call syntax in Test_Executable() previously
included instead from v8.2.2259.

src/nvim/eval.lua

@@ -102,38 +102,38 @@ return {
     diff_hlID={args=2, base=1},
     empty={args=1, base=1},
     environ={},
-    escape={args=2},
+    escape={args=2, base=1},
     eval={args=1, base=1},
     eventhandler={},
-    executable={args=1},
-    execute={args={1, 2}},
-    exepath={args=1},
-    exists={args=1},
+    executable={args=1, base=1},
+    execute={args={1, 2}, base=1},
+    exepath={args=1, base=1},
+    exists={args=1, base=1},
     exp={args=1, base=1, func="float_op_wrapper", data="&exp"},
-    expand={args={1, 3}},
-    expandcmd={args=1},
+    expand={args={1, 3}, base=1},
+    expandcmd={args=1, base=1},
     extend={args={2, 3}, base=1},
-    feedkeys={args={1, 2}},
-    file_readable={args=1, func='f_filereadable'},  -- obsolete
-    filereadable={args=1},
-    filewritable={args=1},
+    feedkeys={args={1, 2}, base=1},
+    file_readable={args=1, base=1, func='f_filereadable'},  -- obsolete
+    filereadable={args=1, base=1},
+    filewritable={args=1, base=1},
     filter={args=2, base=1},
-    finddir={args={1, 3}},
-    findfile={args={1, 3}},
+    finddir={args={1, 3}, base=1},
+    findfile={args={1, 3}, base=1},
     flatten={args={1, 2}},
     float2nr={args=1, base=1},
     floor={args=1, base=1, func="float_op_wrapper", data="&floor"},
     fmod={args=2, base=1},
-    fnameescape={args=1},
-    fnamemodify={args=2},
-    foldclosed={args=1},
-    foldclosedend={args=1},
-    foldlevel={args=1},
+    fnameescape={args=1, base=1},
+    fnamemodify={args=2, base=1},
+    foldclosed={args=1, base=1},
+    foldclosedend={args=1, base=1},
+    foldlevel={args=1, base=1},
     foldtext={},
-    foldtextresult={args=1},
+    foldtextresult={args=1, base=1},
     foreground={},
-    funcref={args={1, 3}},
-    ['function']={args={1, 3}},
+    funcref={args={1, 3}, base=1},
+    ['function']={args={1, 3}, base=1},
     garbagecollect={args={0, 1}},
     get={args={2, 3}, base=1},
     getbufinfo={args={0, 1}},