jsonpath: correctly scan keys with a numeric starting character

When using JSONPath queries with keys that begin with a numeric character, CockroachDB fails to parse them, even when the key is clearly a string and not a numeric literal. This diverges from Postgres, which correctly interprets the key.

Explicitly, this includes handling edge cases like "2e", which resembles scientific notation but should be treated as a string key.

Currently, we're able to work around this problem by quoting the key names (ex. `$."2e"`).

Postgres:
```
norman=# select '$.2e'::jsonpath;
ERROR: 42601: invalid floating point number at or near "2e" of jsonpath input
LINE 1: select '$.2e'::jsonpath;
^
LOCATION: jsonpath_yyerror, jsonpath_scan.l:294
Time: 1.421 ms
norman=# select '$.2ee'::jsonpath;
jsonpath
----------
$."2ee"
(1 row)

Time: 1.056 ms
norman=# select '$.2f'::jsonpath;
jsonpath
----------
$."2f"
(1 row)
```

Cockroach:
```
demo@127.0.0.1:26257/demoapp/defaultdb> select '$.2e'::jsonpath;
ERROR: could not parse "$.2e" as type jsonpath: at or near "invalid floating point literal": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
$.2e
^
demo@127.0.0.1:26257/demoapp/defaultdb> select '$.2ee'::jsonpath;
ERROR: could not parse "$.2ee" as type jsonpath: at or near "invalid floating point literal": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
$.2ee
^
demo@127.0.0.1:26257/demoapp/defaultdb> select '$.2f'::jsonpath;
ERROR: could not parse "$.2f" as type jsonpath: at or near "trailing junk after numeric literal at or near "2f"": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
$.2f
^
```

Jira issue: CRDB-49294