[ddoc preprocessor] Highlight multiple <word> and nested [...] in CLI switches

[ntrel]

Fixes #4421.

Before only the first <> and first [] was matched in the flag string.
Also inner [ chars were wrongly italicised.

Before a space was added before the ] presumably because a tall italic character before runs into it. The trouble with that is it can look weird in other places, adding spaces where there shouldn't be any (e.g. <arch>-<os>, so I removed that.

Update: Also highlight any <word> in helpText even if not in flag string.

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[ntrel]

Before a space was added before the ] ... so I removed that.

Let me know if you'd rather I do that in a separate PR

[ntrel]

BTW removing the filtering is fine because:

• Italicizing <foo> is not dependent on using <foo> in the flag string.
• 'foo' is not actually used for the HTML docs - only for the man page (even though short help is copied sometimes)
• ` is not needed as matching <foo> is now by regex, not by space splitting

[dkorpel]

Looks good as a bug fix, but eventually I want to minimize the amount of special cases and magic transformations used to build the website. I prefer to just write the desired ddoc plainly in the dmd CLI documentation.

[ntrel]

@dkorpel

• I agree that replacing foo in help text when <foo> is in the flag string is bug-prone, I might look at removing that.
• We need some kind of common format e.g. <foo> because this is used to highlight option parameters for the man file. Unless it's possible to make a man.ddoc output target which is compatible with man syntax, but I doubt that would be easy.
• I've just realized I need to fix the man file generation to be consistent with this PR