Add exportName option to rollup plugin

FEATURE: Support an `exportName` option to the Rollup plugin.

Closes #18
See https://discuss.codemirror.net/t/is-there-anyway-to-configure-exportname-when-using-rollup/7594

README.md

@@ -22,4 +22,6 @@ export default {
 }
 ```
 
+The plugin can be passed `lezer({exportName})` option to configure the name of the parser export.
+
 The code is licensed under an MIT license.

src/rollup-plugin-lezer.js

@@ -2,7 +2,7 @@ import {resolve, dirname} from "path"
 import {promises as fs} from "fs"
 import {buildParserFile} from "./index.js"
 
-export function lezer() {
+export function lezer(config = {}) {
   let built = Object.create(null)
 
   return {
@@ -23,6 +23,7 @@ export function lezer() {
       let build = built[base] || (built[base] = fs.readFile(base, "utf8").then(code => buildParserFile(code, {
         fileName: base,
         moduleStyle: "es",
+        exportName: config.exportName,
         warn: message => this.warn(message)
       })))
       return build.then(result => m[2] ? result.terms : result.parser)