Refactor when expression handling

[mjpieters]

• Use expr.Patch and a visitor to only replace identifiers that start with $ with a Env. member lookup.
• use expr.AsBool() to assert the expression produces a boolean
• Clean up envMap parsing to use strings.Cut()
• Expand tests, using testing.T.Run() with test names.

Fixes #599

[mjpieters]

There are a few changes in behavior:

• using undefined variables in an expression now provides a much friendlier error:

  $ tbls --when 'foo'        
  unknown name foo (1:1)
  | foo
  | ^

  Before you got:

  $ tbls --when 'foo'
  cannot fetch foo from struct { Env map[string]string } (1:2)
  | (foo) == true
  | .^

• You now also get an error when the expression isn't going to produce a boolean value:

  $ tbls --when '$SHELL'
  expected bool, but got string

  Without this PR, there was no error, the test simply failed because no string is ever equal to true.

• Accessing an environment variable that doesn't exist results in an empty string (the default 'zero' value for the string type):

  $ tbls --when '$NONESUCH != ""'

  where before this would be a (cryptic) error:

  $ tbls --when '$NONESUCH != ""'
  cannot fetch $NONESUCH from struct { Env map[string]string } (1:2)
  | ($NONESUCH != "") == true
  | .^

  You can use in and not in to test if a variable is defined, instead:

  $ tbls --when '"NONESUCH" in Env`

[k1LoW]

@mjpieters Looks Great!!! thank you!!!