Docs: document E054, E055, E056

Author: strager

docs/errors/E054.md

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+# E054: unexpected token
+
+If you get error E054, quick-lint-js does not understand your code for some
+reasons. The authors of quick-lint-js have not written a helpful message.
+
+If you encounter this error, please [submit a bug
+report](https://github.com/quick-lint/quick-lint-js/issues).
+
+Typically, this error occurs if code contains an operator which doesn't belong:
+
+    let person = "Sam";:
+
+To fix this error, write correct JavaScript syntax:
+
+    let person = "Sam";

docs/errors/E055.md

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+# E055: unmatched indexing bracket
+
+```config-for-examples
+{
+  "globals": {
+    "friends": true
+  }
+}
+```
+
+It is an error to write the indexing operator `[` without its matching `]`:
+
+    for (let i = 0; i < friends.length; ++i) {
+      let friend = friends[i;
+      console.log[(`Hello, ${friend}!`);
+    }
+
+To fix this error, write the closing `]`, or remove the extraneous `[`:
+
+    for (let i = 0; i < friends.length; ++i) {
+      let friend = friends[i];
+      console.log(`Hello, ${friend}!`);
+    }

docs/errors/E056.md

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+# E056: unmatched parenthesis
+
+It is an error to write the `(` without its matching `)`:
+
+    function fma(a, b, c) {
+      return (a * b + c;
+    }
+    let five = (fma(2, 2, 1);
+
+To fix this error, write the closing `)`, or remove the extraneous `(`:
+
+    function fma(a, b, c) {
+      return (a * b) + c;
+    }
+    let five = fma(2, 2, 1);