feat: add chars method for builtin str

[jellllly420]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y fix [Feature Request] Enhance String Handling Consistency for Unicode and ASCII in KCL #1783

2. What is the scope of this PR (e.g. component or file name):

kclvm/runtime/src/
kclvm/runtime/src/value/
kclvm/runtime/src/builtin/
kclvm/tests/test_units/runtime/str/
test/grammar/builtins/str/

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

This PR adds a chars method for builtin str to return a list of the characters in the string.

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

Runtime test: kclvm/tests/test_units/runtime/str/
Grammar test: test/grammar/builtins/str/

[He1pa]

Some lsp test failed about str std method. just re-run and update *.snap file

[He1pa]

cc @zong-zhe

[coveralls]

Pull Request Test Coverage Report for Build 12653107375

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 4 of 21 (19.05%) changed or added relevant lines in 4 files are covered.
• 81 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 70.698%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
kclvm/runtime/src/value/val_str.rs	0	7	0.0%
kclvm/runtime/src/value/api.rs	0	10	0.0%

Files with Coverage Reduction	New Missed Lines	%
kclvm/runtime/src/value/iter.rs	3	81.98%
kclvm/runtime/src/api/utils.rs	12	57.81%
kclvm/sema/src/resolver/config.rs	66	83.52%

Totals
Change from base Build 12370734421:	0.02%
Covered Lines:	52854
Relevant Lines:	74760

---

💛 - Coveralls

[jellllly420]

Sorry for the long delay. I have no clue about the CI failure. I cannot reproduce it locally since the official dev container's configuration is different from the workflow machine and the test suite doesn't work well in the dev container. Furthermore, I didn't find a relevant doc about the load_package's internal data structure. I'd be appreciate if someone could help on this issue. I noticed @He1pa has committed some fix but it still seems not right. ;(

[He1pa]

Sorry for the long delay. I have no clue about the CI failure. I cannot reproduce it locally since the official dev container's configuration is different from the workflow machine and the test suite doesn't work well in the dev container. Furthermore, I didn't find a relevant doc about the load_package's internal data structure. I'd be appreciate if someone could help on this issue. I noticed @He1pa has committed some fix but it still seems not right. ;(

Ok, I will complete this PR later, but not now.
Anyway, thank you for your contribution