feat(transpiler): overhaul formatting and customizability

[AlexTMjugador]

While the current GLSL transpiler output format is quite customizable, I needed to further extend its customizability to better support outputting GLSL in both pretty and minified formats. To that end, these changes overhaul the transpiler's formatting logic and implement some minor format changes and fixes. The fixes included are complementary to those in https://github.com/vtavernier/glsl-lang/pull/19.

I considered splitting this PR into several smaller ones, but decided against it because the changes are so closely related that doing so would make them harder to review. Please let me know your thoughts on this!

The full list of changes, in no particular order, is as follows:

• Added a feature to collapse compound statements with a single statement to the statement itself, but only if the compound statement is not part of a function definition, in which case case the GLSL grammar requires a compound statement. This collapsing feature is toggleable and disabled by default.
• Added formatting options to toggle whether spaces are inserted around open block braces, after case labels, before the else keyword, after the list separator and for separator tokens (comma and semicolon, respectively), around the initializer list expression braces, and around the parentheses that are part of some statements, such as selection, jump, or switch statements.
• Replaced instances of write_str of a single char with write_char.
• Removed a extraneous whitespace before the opening parenthesis of a layout specifier.
• Expanded the set of operator-like tokens written with write_binary_op so that the value of the corresponding format option is applied more consistently.
• Fixed not outputting the semicolon required to terminate the expression statement in a for init statement.
• Removed excess whitespace after the return keyword when no expression follows, which is better for both minified and prettified output.
• Updated tests.
• Added a FormattingSettings::minifying factory method to get formatting settings that minify the generated GLSL output, which is more convenient than setting each option to the most space-saving value.

[AlexTMjugador]

Thanks for the review!

I didn't know that the pretty crate was a thing. In hindsight, it probably would have been a good idea to refactor the transpiler to use it, since it allows a greater degree of abstraction over low-level formatting details, as you've said. But I'm glad that this PR was able to cobble together a better transpiler without resorting to that crate, at least for now :wink: