Prevent overwriting plugins

[asmockler]

Addresses #713

First started looking for a way to fix this because I was bumping into this issue: developit/microbundle#74

Since plugins was hanging out in the global scope, it was possible for plugins to be overwritten. This PR moves the plugins object inside the Parser class to avoid overwriting between parser instances. I've left the global plugins object as well since it is needed here:

acorn/src/index.js

Lines 73 to 78 in 832c308

	export let parse_dammit, LooseParser, pluginsLoose // eslint-disable-line camelcase
	export function addLooseExports(parse, Parser, plugins) {
	parse_dammit = parse // eslint-disable-line camelcase
	LooseParser = Parser
	pluginsLoose = plugins
	}

[marijnh]

I am interested in changing the plugin system to avoid this issue, but I don't think this PR is what we're looking for — it further complicates the way plugins work, without really addressing the underlying issue.

[asmockler]

Is there a proposal or do you have a preferred way you would like the plugin system to be changed? I don't have a lot of context but would be willing to implement if there is a somewhat straightforward implementation plan.

[marijnh]

Unfortunately, there's no plan yet, so the main work would be to design one. I've been kind of distracted from this project but enough stuff has piled that would warrant a breaking new version that I guess I should look into it soon. See also #656. I'll try to make time next week.