tl: extend F[n:m] and G[n:m] to the case of m=$

Suggested by Victor Khomenko.

* spot/tl/formula.cc, spot/tl/formula.hh, spot/parsetl/parsetl.yy:
Implement this.
* NEWS, doc/tl/tl.tex: Document it.
* tests/core/sugar.test, tests/python/ltlparse.py: Add some tests.
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