Fix JSON reader rejecting exponent notation without a fraction

[VXNCXNX]

What's broken

Valid JSON numbers in exponent notation without a decimal point are rejected outright.

Repro

$ echo '{"a":1e3}' | dasel -i json -o json
before: dasel: error: error reading input: could not decode object:
        strconv.ParseInt: parsing "1e3": invalid syntax
after:  {"a": 1000}

$ echo '{"a":1E-2}' | dasel -i json -o json
before: same error on "1E-2"
after:  {"a": 0.01}

2e+2 fails the same way.

The fix

decodeToken only treated a number as a float when the literal contained a ., so the exponent forms went to ParseInt. It now also checks for e or E. Those are the only other characters json.Number can hold, so there is no false positive.

Verification

TestJSONReader_ExponentNumbers covers positive, negative and signed exponents, a fraction with an exponent, and plain integers staying ints. Reverting to the . check fails it on the parse error.

I compared binaries built before and after across the number forms rather than reasoning from the diff: 1, -3, 0, 1.0, 1.5 and an 18 digit integer all produce byte-identical output. Only the three exponent forms change, from an error to a value.

One consequence worth naming: 1e3 now reads as a float, so -o yaml gives !!float 1000. The YAML reader already did exactly that for 1e3 on master, so this aligns the two readers rather than introducing a new behaviour.

go test ./... passes across all 21 packages. gofmt and go vet clean. CHANGELOG entry added under Unreleased/Fixed.