[buildifier] Proper handling of scientific notation in arguments

Currently,
```
tolerance = 5e-3
```

gets formatted to
```
tolerance = 5e - 3
```

as if the numbers were being subtracted. This is less clear than keeping the characters all adjacent to each other.

To reproduce the issue, clone the repo. Then run:
```
$ echo 'filegroup(name="foo", tolerance = 5e-3)' > BUILD.bazel
$ cat BUILD.bazel
filegroup(name="foo", tolerance = 5e-3)
$ buildifier BUILD.bazel
$ cat BUILD.bazel
filegroup(
name = "foo",
tolerance = 5e - 3,
)
```

Reproduce the issue with the BUILD.bazel example and the `buildifier BUILD.bazel` command from the report. Then trace how buildifier formats scientific-notation arguments; done means `5e-3` remains adjacent after formatting rather than becoming `5e - 3`, with the existing formatting behavior preserved elsewhere.

Written by the indexing model from the issue text.