Fixes the following error:

    src/ShellCheck/CFGAnalysis.hs:1394:40: error:
	* Couldn't match expected type `[S.Set a]'
		      with actual type `M.Map String FunctionValue'
	* In the second argument of `($)', namely
	    `mapStorage $ sFunctionTargets state'
	  In the expression: S.unions $ mapStorage $ sFunctionTargets state
	  In an equation for `declaredFuncs':
	      declaredFuncs = S.unions $ mapStorage $ sFunctionTargets state
	* Relevant bindings include
	    declaredFuncs :: S.Set a
	      (bound at src/ShellCheck/CFGAnalysis.hs:1394:13)

Permit colon after exec

This reverts commit 5202072a.

Fix false positive for SC2312 when using `time`

ShellCheck throws warning SC2093 when a script contains commands that could never be executed because they are after an `exec`.  Command `:` does nothing, so add it to the list of commands that don’t trigger this warning.

Support more Bash internal variables

Suppress SC2311 with `set -o posix`

Add `mapfile` to harmless commands for SC2094

  - BASH_ARGV0, introduced in Bash 5.0
  - BASH_COMPAT, 4.3
  - BASH_LOADABLES_PATH, 4.4
  - CHILD_MAX, 4.3
  - EPOCHREALTIME, 5.0
  - EPOCHSECONDS, 5.0
  - EXECIGNORE, 4.4
  - INSIDE_EMACS, 4.4
  - PS0, 4.4
  - READLINE_ARGUMENT, 5.2
  - READLINE_MARK, 5.1
  - SRANDOM, 5.1

Fixes #1780 and #2554.

No functional changes; this just makes the next few commits cleaner.

This fixes `Prelude.foldl1: empty list []` when script has `( exit )`