patch 9.0.1958: cannot complete option values

Problem:  cannot complete option values
Solution: Add completion functions for several options

Add cmdline tab-completion for setting string options

Add tab-completion for setting string options on the cmdline using
`:set=` (along with `:set+=` and `:set-=`).

The existing tab completion for setting options currently only works
when nothing is typed yet, and it only fills in with the existing value,
e.g. when the user does `:set diffopt=<Tab>` it will be completed to
`set diffopt=internal,filler,closeoff` and nothing else. This isn't too
useful as a user usually wants auto-complete to suggest all the possible
values, such as 'iblank', or 'algorithm:patience'.

For set= and set+=, this adds a new optional callback function for each
option that can be invoked when doing completion. This allows for each
option to have control over how completion works. For example, in
'diffopt', it will suggest the default enumeration, but if `algorithm:`
is selected, it will further suggest different algorithm types like
'meyers' and 'patience'. When using set=, the existing option value will
be filled in as the first choice to preserve the existing behavior. When
using set+= this won't happen as it doesn't make sense.

For flag list options (e.g. 'mouse' and 'guioptions'), completion will
take into account existing typed values (and in the case of set+=, the
existing option value) to make sure it doesn't suggest duplicates.

For set-=, there is a new `ExpandSettingSubtract` function which will
handle flag list and comma-separated options smartly, by only suggesting
values that currently exist in the option.

Note that Vim has some existing code that adds special handling for
'filetype', 'syntax', and misc dir options like 'backupdir'. This change
preserves them as they already work, instead of converting to the new
callback API for each option.

closes: #13182

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

src/vim.h

@@ -822,6 +822,8 @@ extern int (*dyn_libintl_wputenv)(const wchar_t *envstring);
 #define EXPAND_BREAKPOINT	51
 #define EXPAND_SCRIPTNAMES	52
 #define EXPAND_RUNTIME		53
+#define EXPAND_STRING_SETTING	54
+#define EXPAND_SETTING_SUBTRACT	55
 
 // Values for exmode_active (0 is no exmode)
 #define EXMODE_NORMAL		1
@@ -2330,6 +2332,20 @@ typedef enum {
  */
 typedef char *(*opt_did_set_cb_T)(optset_T *args);
 
+/*
+ * Type for the callback function that is invoked when expanding possible
+ * string option values during cmdline completion.
+ *
+ * Strings in returned matches will be managed and freed by caller.
+ *
+ * Returns OK if the expansion succeeded (numMatches and matches have to be
+ * set). Otherwise returns FAIL.
+ *
+ * Note: If returned FAIL or *numMatches is 0, *matches will NOT be freed by
+ * caller.
+ */
+typedef int (*opt_expand_cb_T)(optexpand_T *args, int *numMatches, char_u ***matches);
+
 // Flags for assignment functions.
 #define ASSIGN_VAR	0     // ":var" (nothing special)
 #define ASSIGN_FINAL	0x01  // ":final"