feat: Add semantic linting for Splunk .conf files

[jtgreen86]

What this adds

Semantic linting for Splunk .conf files: context-aware configuration suggestions that go beyond syntax validation. Traditional linting only checks if stanzas and settings are valid per .spec files. This feature detects issues that are syntactically valid but problematic in practice.

Problems it catches

• Performance anti-patterns (e.g. SHOULD_LINEMERGE = true when LINE_BREAKER would be faster)
• Missing complementary settings (e.g. LINE_BREAKER without SHOULD_LINEMERGE = false)
• Conflicting configurations (e.g. KV_MODE = json with INDEXED_EXTRACTIONS)
• Risky values (e.g. TRUNCATE too low causing data loss)
• Incomplete configurations (e.g. TIME_FORMAT without TIME_PREFIX)

Files added

File	Purpose
out/semanticRules.js	Rule loading and evaluation engine
out/codeActionProvider.js	Quick fix code actions
resources/semantic_rules.json	Rule definitions (JSON, not hardcoded)
resources/semantic_rules.schema.json	JSON Schema for rule validation

Files modified

File	What changed
out/extension.js	Added imports, registered code action provider, integrated semantic linting into getDiagnostics()
package.json	Added splunk.semanticLinting.enabled configuration setting
README.md	Added semantic linting to feature list
CHANGELOG.md	Added version 0.4.0 entry

Notes

• Rules are JSON-driven and easy to extend without code changes
• Severity levels are appropriate (Error/Warning/Information/Hint)
• Feature can be disabled via the splunk.semanticLinting.enabled setting
• No existing functionality was modified (syntax validation, completions, etc. unchanged)