To preserve backward compatibility with the old parser which would
ignore syntax errors, this PR introduces a way to treat the special
characters `{` and `}` in the following way:
1. If we encounter a `{` which searching for a string token and this `{`
is not followed by a valid placeholder, then we treat the `{` as a
string and continue lexing for strings.
2. If we encounter a `}` while not within some expression (i.e.
placeholders, arguments, plurals, or selects), then we treat the `}` as
a string and continue lexing for strings.

This makes it so that
```
"helloWorld": "{ } { placeholder }",
"@@helloWorld": {
  "placeholders": {
    "placeholder" {}
  }
}
```
treats the `{ }` as a string while `{ placeholder } ` is treated as a
placeholder.

Fixes https://github.com/flutter/flutter/issues/122404.