Literals which are unquoted strings not being verified for JSON syntax

My understanding based on [the specification](https://jmespath.org/specification.html#literal-expressions) is that the content of a literal expression, i.e. the text between backticks, is supposed to be a valid JSON expression, otherwise it should be reported as a syntax error. The Python implementation seems to be treating them as strings instead, though. For example, this is accepted as a valid JMESPath expression:
```
thing == `wat`
```
even though I would think it should cause an error. Is this an actual bug in the Python implementation, or have I been misunderstanding the spec? (In the latter case I'd argue that the spec is misleading about this to the point of it being a documentation bug, but that's a separate issue.)

Here's a short script that demonstrates the error:
```python
import jmespath, json, sys

literal_json = 'wat'
expression = f"thing == `{literal_json}`"
document = {"thing": "wat"}

try:
r = json.loads(literal_json)
except Exception as e:
print(f"json: parse error: {e.__class__.__name__}")
else:
print("json: no parse error")
print(f" result: {r!r}")

try:
r = jmespath.search(expression, document)
except Exception as e:
print(f"jmespath: parse error: {e.__class__.__name__}")
else:
print("jmespath: no parse error")
print(f" result: {r!r}")
```
It prints out the following:
```console
$ python test-jmespath.py
json: parse error: JSONDecodeError
jmespath: no parse error
result: True
```
I would expect to see the `jmespath` part of the output be the same as the `json` part of the output, i.e. some kind of parsing error. And if I try other kinds of values, I do indeed get consistent results. For example, replacing the `literal_json` assignment line with `literal_json = '3'` gives
```console
$ python test-jmespath.py
json: no parse error
result: 3
jmespath: no parse error
result: False
```
and if I replace it with `literal_json = '{"foo": bar}'` I get
```console
$ python test-jmespath.py
json: parse error: JSONDecodeError
jmespath: parse error: LexerError
```