The consequence of this change is that if `syntect` encounters a syntax
that references another syntax that is not present, `syntect` will
fallback to Plain Text syntax instead of erroring/panicking. Falling
back to Plain Text in cases like this seems to be what Sublime Text is
doing too.

For example, `bat` has a syntax for Vue, but Vue references a syntax
with scope `text.pug`, which is not included in bat (by default).

So if `bat` encounters the following `vue-with-pug.vue` file:
```
<template lang='pug'>
  #container.col
    p This shall be formated as Plain Text as long as a Pug syntax definition is missing
</template>
```
`bat` currently panics with:

```
% bat ~/Desktop/vue-with-pug.vue
───────┬────────────────────────────────────────────────────────────────
       │ File: /Users/martin/Desktop/vue-with-pug.vue
       │ Size: 140 B
───────┼────────────────────────────────────────────────────────────────
thread 'main' panicked at 'Can only call resolve on linked references: ByScope { scope: <text.pug>, sub_context: None }', /Users/martin/.cargo/registry/src/github.com-1ecc6299db9ec823/syntect-4.6.0/src/parsing/syntax_definition.rs:191:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

With this change, instead of panicking, `bat` will print the file with
the pug parts highlighted as Plain Text.