Revamp how documentation tooltips work

[YeldhamDev]

This PR does the following this:

• Add a set of get_*_description() functions to EditorHelpBit, that fetches the doc description of various components, and then caches them.
• Create EditorHelpTooltip, that handles the doc tooltips that before were scattered across multiple places. It internally uses the functions above.
• As a bonus, prettify a little some simple uses of EditorHelpBit I found while making this PR.

This PR is necessary for #81284 (see #81284 (comment)).

[YeldhamDev]

@dalexeev Changes made.

[YuriSizov]

Some changes are still required, but overall this should be a great improvement.

We could probably reduce code duplication even more around the whole "No description available for %s." self-modulation construction. But this can be done in a follow-up.

[akien-mga]

Tested this PR briefly while trying to assess whether it would conflict with #76147.

I noticed this bug when you hover a category (@export_category):

ERROR: Invalid tooltip formatting. The expect string should be formatted as 'type|class|property|args'.
   at: parse_tooltip (./editor/editor_help.cpp:2847)

This happens whether the category has a doc hint or not, for example:

extends Node2D

## The script is described here.

## Hello some int
@export var test: int = 6

## My categories are exquisite.
@export_category("Yo cat")

## This is a plop
@export var plop: int = 42

For the record, in 4.2-dev5 categories don't seem to support doc hints in the tooltip, they always show the category's name as a tooltip. In this PR, it shows an empty tooltip + the error mentioned above.

Ideally it would be good for categories to support doc hints too, but that's probably for a separate PR.

Tested @export_subgroup too for good measure, those seem to work like before this PR (no support for doc hints, but shows the group name in the tooltip).

[YuriSizov]

LGTM, thank you for taking a crack at this refactor

[akien-mga]

Thanks!