Introduce Schematics Tree methods for readText and readJSON

[clydin]

The schematics Tree now contains an additional readText method that supports directly
reading the content of a file as UTF-8 text. This avoids the need to manually decode a Buffer
within a schematic when text content is needed. If a file path does not exist, an exception
will be thrown. While this differs from the semantics of read, it helps reduce the amount
of code needed for common schematic use cases.

The schematics Tree now contains an additional readJSON method that supports directly
reading and parsing the content of a file as UTF-8 JSON with comment support. This avoids the need to manually
decode a Buffer, parse and handle JSON comments within a schematic when JSON content is needed.
If a file path does not exist, an exception will be thrown. While this differs from the semantics of read,
it helps reduce the amount of code needed for common schematic use cases. JSON parse errors will also result
in an exception being thrown with a message detailing the error.

[dgp1130]

Thanks for splitting up the commits and taking the time to update existing usages. The readJSON() improvements seem particularly nice.

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