feat: add api list_variables to select variable

[zong-zhe]

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

• N
• Y

fix #1191

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

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

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

[coveralls]

Pull Request Test Coverage Report for Build 8647168386

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

• 469 of 525 (89.33%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 71.451%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
kclvm/api/src/service/jsonrpc.rs	0	7	0.0%
kclvm/query/src/selector.rs	212	261	81.23%

Files with Coverage Reduction	New Missed Lines	%
kclvm/tools/src/LSP/src/state.rs	1	86.15%

Totals
Change from base Build 8631441282:	0.2%
Covered Lines:	47559
Relevant Lines:	66562

---

💛 - Coveralls

[Peefy]

LGTM