Config path quote handling: keys with quotes (#40976)

As observed in #40944, when using `spack config add <path>`, the `path` might
contain keys that are enclosed in quotes.

This was broken in https://github.com/spack/spack/pull/39831, which assumed that
only the value (if present, the final element of the path) would use quotes.

This preserves the primary intended behavior of #39931 (allowing ":" in values when
using `spack config add`) while also allowing quotes on keys.

This has complicated the function `process_config_path`, but:
* It is not used outside of `config.py`
* The docstring has been updated to account for this
* Created an object to formalize the DSL, added a test for that, and
  refactored parsing to make use of regular expressions as well.
* Updated the parsing and also updated the `config_path_dsl` test with an explicit check.
  At a higher level, split the parsing to check if something is either a key or not:
  * in the first case, it is covered by a regex
  * in the second, it may be a YAML value, but in that case it would have to be the last
    entry of x:y:z, so in that case I attempt to use the YAML handling logic to parse it as such

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