feat(switch): layer in bool expr

[jtroo]

Describe your changes. Use imperative present tense.

Closes #1026

Checklist

• Add documentation to docs/config.adoc
  • Yes
• Add example and basic docs to cfg_samples/kanata.kbd
  • Yes
• Update error messages
  • Yes
• Added tests, or did manual testing
  • Yes

[jtroo]

@oblitzitate can you see if this PR implements the feature as intended?

Example syntax from test file:

(defcfg)
(defsrc a b)
(defalias b (switch
    ((layer base)) x break
    ((layer other)) y break))
(deflayer base (layer-while-held other) @b)
(deflayer other XX @b)

[oblitzitate]

I pretty much applied it similarly to the solution I mentioned in the feature request and it seems to work as intended. Thanks!