Tighten Lambdapi pattern

[nickswalker]

Would match "symbol" or "rule" anywhere in a file, which tripped for plenty of Linear Programs and Answer Set Programs. Now only matches if in a valid lambdapi statement (e.g. symbol ℕ : TYPE; or rule $x + zero ↪ $x;).

Added the sample file that is currently misclassified

Checklist:

• I am fixing a misclassified language
  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • space-error-messages.lp
    • Sample license(s): MIT or Apache-2.0 (see inline SPDX comment^)
  • I have included a change to the heuristics to distinguish my language from others using the same extension.