strings: add find_between_pair

[Larpon]

This PR adds a function I've personally missed a lot while working with strings in V. It introduces a way to extract content between two marks in a string in a "balanced" way. It makes it possible to extract "block" or "scope" content in a way that is currently not possible with either of the find_between, all_after*, all_before* methods defined on the string type.

My personal use case has been mostly to extract blocks of enum or struct contents from C (and V) code.
E.g. getting the contents between the opening { and closing } of the SDL_RWops struct in the following was not easy before without extensive parsing (or position/format hacks):

typedef struct SDL_RWops
{
...
    int (SDLCALL * close) (struct SDL_RWops * context);
    Uint32 type;
    union
    {
#if defined(__ANDROID__)
        struct
        {
            void *fileNameRef;
            void *inputStreamRef;
            ...
            int fd;
        } androidio;
#elif defined(__WIN32__)
        struct
        {
            SDL_bool append;
            void *h;
            struct
            {
                ...
            } buffer;
        } windowsio;
#endif
        struct
        {
            void *data1;
            void *data2;
        } unknown;
    } hidden;
} SDL_RWops;

It's also good for things like (nested) block comments:

/*
 /* hello */
*/

... and for grabbing various parts of V function signatures like:
fn fn_with_anon_fn(i int, callback fn (num int, s string)) <- arguments can be grabbed quickly (including the callback signature) with:
arg_str := strings.find_between_pair(signature,'(',')') // i int, callback fn (num int, s string)

I decided to use a sumtype as mark types as both the performance and use-cases differ for the three types I've had use for:
(10000 iterations)

The benchmark I've used is included here: bmark_find_between_pair.zip

I guess it can be discussed what results should be if the input/marks is "unbalanced"/missing marks/uneven/the same:
I.e what should the result be in the following input cases?:
strings.find_between_pair('( blank ( or full ( or half of it','(','') == ??
strings.find_between_pair('* this part * or this *','*','*') == ??
These can be decided later on - right now these will result in blank or "undefined" (untested) return output.

[Larpon]

Vinix build error is unrelated I think

[Larpon]

Ditto dynamic library example 🤷

[spytheman]

Ditto dynamic library example shrug

Yes, it was a bug in the v.markused module (the functions needed for a#[] were removed with -skip-unused).
Fixed in V f8bf3db .

[Larpon]

Ready when CI is