Fix formatting hex float literals

[mhauru]

Closes #921

There's still this case, where the formatter changes the value of the expression:

julia> format_text("0x1p1.2")
"0x1p10.2"

julia> 0x1p1.2
0.4

julia> 0x1p10.2
204.8

However, I think 0x1p1.2 should be a syntax error, so maybe not worth fixing? I've reported it as a Julia bug here JuliaLang/julia#60189. To be clear, what happens there is that Julia parses 0x1p1.2 as a juxtaposition of 0x1p1 and .2, i.e. a multiplication. The formatter then expands the .2 to be 0.2, which results in 0x1p10.2, changing the value of the first literal that is being multiplied.

[domluna]

lgtm anything else before merging?

[mhauru]

Not from my side, happy to merge.

[mhauru]

Also, the Julia issue I described in the OP got fixed already: JuliaLang/julia#60199