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/structs.h

@@ -4914,6 +4914,34 @@ typedef struct
     char	*os_errbuf;
 } optset_T;
 
+/*
+ * Argument for the callback function (opt_expand_cb_T) invoked after a string
+ * option value is expanded for cmdline completion.
+ */
+typedef struct
+{
+    // Pointer to the option variable. It's always a string.
+    char_u	*oe_varp;
+    // The original option value, escaped.
+    char_u	*oe_opt_value;
+
+    // TRUE if using set+= instead of set=
+    int		oe_append;
+    // TRUE if we would like to add the original option value as the first
+    // choice.
+    int		oe_include_orig_val;
+
+    // Regex from the cmdline, for matching potential options against.
+    regmatch_T	*oe_regmatch;
+    // The expansion context.
+    expand_T	*oe_xp;
+
+    // The full argument passed to :set. For example, if the user inputs
+    // ':set dip=icase,algorithm:my<Tab>', oe_xp->xp_pattern will only have
+    // 'my', but oe_set_arg will contain the whole 'icase,algorithm:my'.
+    char_u	*oe_set_arg;
+} optexpand_T;
+
 /*
  * Spell checking variables passed from win_update() to win_line().
  */