Refactor "gen-l10n" command to use same code path when "l10n.yaml" is present or not present (#125429)

I think this is a long needed change to the `gen-l10n` command.
Essentially, the arguments to `flutter gen-l10n` can be provided by two
different methods: via command line arguments or via the `l10n.yaml`
file. The existence of a `l10n.yaml` file causes the latter approach to
take precedence.

However, currently, there's several differences in how the two
approaches are handled, and most of the default arguments are all over
the place, causing unexpected issues such as #120457 or #120023.

This PR refactors the command so that
* `LocalizationOptions` are more consistent with the actual argument
names/yaml options.
* All default values are determined in `LocalizationOptions`'s
constructor (or in `argParser.addOption(...)` in the case a boolean
value needs to be explicitly true).
* New `parseLocalizationsOptionsFromCommand` function to parse
arguments.
* Parse `LocalizationOptions` at the beginning of `runCommand()` and
pass it to `generateLocalizations`.

Fixes #120023.

Currently, the code doesn't properly handle strings which contain dollar signs. The return expression for the generated localization function is computed by `generateReturnExpr` which concatenates several strings, which are either interpolated placeholders, interpolated function calls, or normal strings, but we didn't properly escape dollar signs before sending normal strings to `generateReturnExpr`.

Fixes #125461.

`intl` [0.18.0 added](https://pub.dev/packages/intl/changelog#0180) a
[`NumberFormat.decimalPatternDigits`](https://pub.dev/documentation/intl/latest/intl/NumberFormat/NumberFormat.decimalPatternDigits.html)
pattern that allows `decimalDigits` param.

Fixes #122785

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Remove 1745 decorative breaks

[flutter_tools] toolexit on malformed yaml during gen-l10n

* lerp documentation

* Remove Note, Note That from repo

* Improve BorderSide documentation.

* apply review comments

* init

* fix error handling

* fix issue

* lint?

* error handling tests

* lint

*  Do not format the messages file

*  Add test

*  Add the link to the test

*  Respect the path separator from the file system

*  Add the untranslated messages file existence test

* init

* lint

* lint again

* Remove superfluous words from comments.

* Revert `win32_window.h`

Which seems to come from `gallery`

* dart fix --apply

* manual fixes

* fix after merge conflicts

* review

* init

* add comment

* make error more actionable

* init

* empty commit to start google testing

* [gen_l10n] keys can contain dollar sign

Fixes #112250

* Update packages/flutter_tools/lib/src/localizations/gen_l10n.dart
Co-authored-by: Christopher Fujino <fujino@google.com>

* init

* fix tests

* fix lint

* extra changes

* oops missed some merge conflicts

* fix lexer add tests

* consistent warnings and errors

* throw error at the end

* improve efficiency, improve code generation

* fix

* nit

* fix test

* remove helper method class

* two d's

* oops

* empty commit as google testing won't pass :(

* init

* make more descriptive

* fix lint

* init

* more fixing

* finish

* fix lint

* address pr comments

* redo checks

* init

* code generation

* improve syntax error, add tests

* add tests and fix bugs

* code generation fix

* fix all tests :)

* fix bug

* init

* fix all code gen issues

* FIXED ALL TESTS :D

* add license

* remove trailing spaces

* remove print

* tests fix

* specify type annotation

* fix test

* lint

* fix todos

* fix subclass issues

* final fix; flutter gallery runs

* escaping for later pr

* fix comment

* address PR comments

* more

* more descriptive errors

* last fixes

This reverts commit f8f69575.

* Migrate packages/flutter_tools/bin

* Fix NPE

* Fix test

* Fix test

* Fix l10n optionalParameters

* Fix ChromeTab nullability

* Fix another type cast error

* Fix another cast error

* Fix another cast error (copied from #110711)

* Fix NPE

* Fix another NPE

* Assert that testDirectory is not null

* init

* fix

* fix 2

* fix 3

* tests

* fix tests

* clarify help text

* fix all tests

* fix formatting?

* add second test

* unused import

* remove print

* trailing spaces

* artifacts is never null

* fix

[gen_l10n] Warn users when placeholder types are converted to 'num' when using pluralization (#108036)

* init

* asdf

* fix tests

* change warning slightly

* fix test again

* fix spacing and comment

* fix lint

This reverts commit 16f44118.