[lex] Complete the use of unicode code points

[AlisdairM]

Completes the task of applying Unicode code point markup to
refer to single characters in normative text.

Replace all remaining uses of backslash as glyph () or text
with the corresponding unicode{005c}{reverse solidus} markup.

Replace a couple of text reference to single and double quotes
with their corresponding Unicode markup, and similarly for the
glyphs ` and ".

This should compete the last Unicode markup in [lex].

Resolves #5502

[eisenwave]

Making this P2 because the direction affirmed for P2843R3 is that we ought to use code points instead of colloquial names like "backslash".

[tkoeppe]

Could you reword the commit message to describe the change made by the commit, instead of the wider project (which isn't very descriptive)?

[jensmaurer]

@AlisdairM , did you intend to close this?

[AlisdairM]

No, I pushed an update that went to the wrong repo --- I am disconnected.

[jensmaurer]

Ack. I had to make this repo private due to the other pull request about ongoing business, and that disconnected all forks. Quite a fall-out.

[tkoeppe]

@AlisdairM: I reopened this.

[AlisdairM]

I believe that this PR should be visible and open again, with the revised comment message that Thomas asked for, or at least my attempt at such a rewording.

[jensmaurer]

Looks good, but the commit message needs a bit of work. "Complete X" is not descriptive of the actual changes made in this commit.

[tkoeppe]

@AlisdairM Please rebase

[AlisdairM]

Rebased, and should be ready to merge.