add array method `last_index()`

[kbkpbot]

Describe the feature

Just like a string, I think we need a last_index() for array too, currently, array has a method index()

This is a feature request by fixing issue #26243

Use Case

For string builder work for searching a buffer from the end.

mut sb := strings.new_builder(100)
....
if sb.last_index('=') {
 ....
}

Proposed Solution

add a method last_index() for array type.

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

V 0.5.0 49bade5

Environment details (OS name and version, etc.)

V full version	V 0.5.0 49bade5
OS	linux, Ubuntu 24.04.3 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz
Memory	10.52GB/15.51GB
V executable	/media/HD/github/kbkpbot/v/v
V last modified time	2026-01-02 14:37:03
V home dir	OK, value: /media/HD/github/kbkpbot/v
VMODULES	OK, value: /home/mars/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/mars/v/bug/arr
Git version	git version 2.43.0
V git status	weekly.2025.52-109-g49bade56-dirty
.git/config present	true
cc version	cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
gcc version	gcc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
clang version	Ubuntu clang version 18.1.3 (1ubuntu1)
tcc version	tcc version 0.9.28rc 2025-02-13 HEAD@f8bd136d (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 696c1d84
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.39-0ubuntu8.6) 2.39

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