parsing snippets now first involves attempting to import it as a javascript
module, with the default export expected to be the snippets.
this enables improved snippet config writing, and could supercede the need for
the "variables" setting of the plugin.

if importing fails, falls back to existing behavior of parsing as json5 string.

security risk: dynamically imported user-provided files may execute malicious
code; users should be validate snippet files others share.

… flags

in js snippets files, support passing in regexp literals as the trigger.
also, generally support regexp flags for regexp snippets (i.e. regexp literals,
or snippets with the `r` option).
only the following flags are allowed: `i`, `m`, `s`, `u`, `v`.
when a snippet with a regexp literal trigger also has flags defined,
the flags get merged together.

describe snippet file formats (js, json5); add documentation on added `flags`
field; note validity of regex triggers in js snippet files.

previously, flags were marked as optional field.
however, flags are always defined as a string, even if an empty one.

add stricter validation checks, and improve validation check type signature

removes the dependency on json5.
if the snippets file isn't a js module with a default export,
it now gets treated as a js array via an `export default ` prepended to it,
as opposed to a json5 array.
json5 is valid js so it should hopefully not break anything

update docs to not mention json5 since it's no longer used.

add a warning on readme underneath snippet sharing,
to warn about the dangers of running arbitrary javascript code.

typo in code was using unmodified raw options string,
so regex literal triggers did not result in `Option.regex` being marked.

previously was a hardcoded color that was not very readable on dark bg

remove redundant uses of raw snippet, make state of values more explicit.

`RawSnippet` is never actually instantiated, and serves more so as an interface
representing the expected shape of a parsed snippet. as such an interface is
more appropriate than a `class` for it.

in `ParsedSnippet` constructor, variable was named `override` because it
overrode certain fields of `raw` in construction of final parsed snippet.
however, `parsed` no longer uses both `raw` and `override`, and so as such
`override` more so represents the _resolved_ fields.

…andled