Fall back to 'Plain Text' if a referenced syntax is missing

[Enselic]

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:

Closes #421

[Enselic]

I tried putting this logic inside of Self::find_id() directly, which would simplify the code. But rustc complained about reached the recursion limit while instantiating 'func::<[closure]>'. This is because the predicate parameter is generic, and calling find_id() inside of itself causes recursion without a base case. This problem is explained in detail here.

This was the most straightforward way I could come up with to work around that.

[keith-hall]

I was wondering about that, thanks for the explanation :)