mermaid from imported markdown files

[azatoth]

Describe the bug
When importing a file into typedoc using [[include:path/to/file.md]], mermaid tags will not be handled and it will complain it can't highlight it.

To Reproduce

Steps to reproduce the behavior:

1. Create typescript foo.ts:

/**
 * [[include:foo.md]]
 */
export class Foo {

}

2. Create foo.md with:

# Foo

```mermaid
  flowChart TB
    A --> B
```

3. Execute typedoc:
   typedoc src/foo.ts --logLevel Verbose --includes src/

Info: Loaded plugin typedoc-plugin-mermaid
Debug: Using TypeScript 4.3.5 from ./node_modules/typescript/lib
Debug: Converting with 1 programs 1 entry points
Debug: Finished getting entry points in 7598ms
Debug: Begin readme search at ./src
Debug: Finished conversion in 99ms
Debug: Renderer: Loading highlighter took 472ms
Warning:
Unsupported highlight language "mermaid" will not be highlighted. Run typedoc --help for a list of supported languages.
target code block :
flowChart TB
A --> B
source files :undefined
output file :
undefined
Info: Documentation generated at ./docs
Debug: HTML rendering took 493ms

4. See error

Expected behavior
I expect mermaid blocks be rendered from the included markdown file

[kamiazya]

@azatoth
It didn't seem to work because the Markdown conversion and file loading timings didn't match.

PR Welcome.

[kamiazya]

fixed by #560

Thanks for @WHenderson !