cmListFileLexer: Fix lexing of single '[' character (#15092)

The lexer changes in commit v3.0.0-rc1~495^2 (Add Lua-style long
brackets and long comments to CMake language, 2013-08-06) accidentally
left out matching '[' as a single character in an unquoted argument.
Add a lexer rule to match it and extend the RunCMake.Syntax test to
cover this case.
Merge branch 'fix-lexer-open-square' into release