feat: add builtin types/functions to diagnostics and completions

Files Modified/Created:
- src/builtin_instructions.lua - NEW: Contains all TIA Portal built-in types, FBs, and functions
- src/diagnostics.lua - Added missing data types + builtin instruction checking
- src/main.lua - Added completion support for built-in types/F Bs/functions
- src/plc_json.lua - Added .db file parsing + BOM handling fixes
- lua/scl/blink_cmp_source.lua - Extended completion with all built-in types/F Bs/functions
- src/node-types.json - Added node types for parser
- AGENTS.md - Updated documentation
Features Added:
1. Diagnostics - No more false "Unknown data type" warnings for:
   - All elementary types (USINT, SINT, UINT, UDINT, LINT, ULINT, TOD, DTL, etc.)
   - Timer/counter types (IEC_TON, TOF, TP, CTU, CTD, CTUD)
   - TIA Portal built-in FBs and functions
2. Auto-completion - Shows in VAR section after ::
   - 28 elementary data types
   - 23 built-in function blocks (TON, TOF, TP, CTU, CTD, CTUD, R_TRIG, F_TRIG, etc.)
   - 80+ built-in functions (ADD, SUB, MUL, DIV, SIN, COS, SQRT, etc.)

src/node-types.json

@@ -1524,10 +1524,18 @@
     "type": "tod",
     "named": false
   },
+  {
+    "type": "TOD",
+    "named": false
+  },
   {
     "type": "uint",
     "named": false
   },
+  {
+    "type": "UInt",
+    "named": false
+  },
   {
     "type": "usint",
     "named": false
@@ -1552,4 +1560,4 @@
     "type": "}",
     "named": false
   }
-]
+]